Hello, I am a regular (not weekend) beta tester. I am planning to main a healer, so I gave all three a chance. This is a comparison of the healers, based on what I've played and what other testers have said.
There are three healing ACs (advance classes) in the game per faction. They are Operative, Sorceror, and Mercenary on the Empire, and Scoundrel, Sage, and Commando on Republic. If you are unfamiliar with the process, you start the game as a class (i.e. Jedi Counsular). At level 10, you pick an advance class (i.e. Sage), and you put talent points into one of three trees (i.e. Seer) unique to that AC.
You can see all the talent trees here: http://www.torhead.com/calculator/skill
Note: For brevity, I will be using the Empire names for classes and abilities. If you want to play Republic, don't worry, it's the same. You'll just need to cross-check the names of the abilities.
Note 2: They are rapidly improving all the classes, so I expect by release, the gap between the healers will be much smaller. Don't worry too much about which healer heals the most. Instead, you should think about which playstyle fits you the best.
Operative / Scoundrel
Base Class: Imperial Agent / Smuggler
Healing Talent Tree: Medicine / Sawbones
Closest WoW Equivalent: Holy Paladin (has mechanics reminiscent of holy power), Resto Druid (HoTs and AoE HoTs are a major focus)
Equipment: Medium Armor
Primary Stat: Cunning
Unique Mechanic: "Tactical Advantage"
Talent Trees: Operatives (and Scoundrels) are the Frankensteins of healers. If you took the healing abilities of a paladin and gave them to a rogue, but left some medium-ranged attacks for good measure, you would have an operative. Two of the talent trees emphasis stealth-based melee dps, while the third talent tree is focused on healing.
Resource: Operatives use ENERGY for their resource, which is functionally identical to the Bounty Hunter's heat and Trooper's ammo. All these classes are restrained by a limited resource pool. Energy regenerates on a GATED (tiered) system. The less energy you have, the less regen you will have (2 to 5 energy/sec). So the best strategy is to maintain high energy levels so that you have high regen levels. When I maintained maximum regen and maintained the stim boost self-buff, I was able to keep a group alive without ever going oom.
Unique Mechanic: Tactical advantage works similarly to holy power. Certain abilities (direct heals, hot ticks, some attacks) give you a point of Tactical Advantage. Unlike holy power, abilities that consume tactical advantage only consume ONE point. So you can save it up and use it one-by-one. Most abilities that consume tactical advantage do not use energy, so it's important to rotate TA-generating and TA-consuming abilities so that you regen stays high.
The regular heals are:
1. Kolto Injection: 2.5 cast (2.0 after talents) heal, 25 energy (can generate TA with talents)
2. Kolto Probe: Instant HoT, stackable, 15 energy (can generate TA with talents)
3. Recuperative Nanotech: Instant AoE HoT, 30 energy, has cooldown
4. Diagnostic Scan: 3.0 sec channel, 0 energy*
*: (this is a very weak heal, but it can be talented to regen your energy when it crits, so it acts as nice filler)
The TA-requiring heals are:
1. Kolto Infusion: 1.5 cast heal, 20 energy, consumes TA
2. Surgical Probe: Instant heal, 0 energy, consumes TA
Other Healer Abilities:
1. Resuscitation Probe: Combat rez
2. Toxin Scan: Instant, Removes debuffs (can be talented to provide direct heal, too)
3. Stim Boost: Regenerates energy, consumes TA. Keep this active all the time.
In my opinion, operatives and scoundrels can be a lot of fun if you like a dynamic healing rotation. You can only spam your bread-and-butter about 6 times before going oom. Therefore, to maintain a good energy efficiency, you're better off acting like a druid most of the time. Like druids, operatives and scoundrels spend most of their "resting" time by maintaining HoTs on their allies. If the fight doesn't become stressful, your stacking HoT (plus the AoE HoT if necessary) can keep everyone alive without difficulty. The HoT gives you points of TA, so you can intersperse your HoT ticks with the instant, free heal.
Once the fight comes stressful (boss), you go into paladin-mode and must wisely triage your direct heals by switching between Kolto Infusion (cheaper, needs TA), Kolto Injection (expensive, makes TA), and Surgical Probe (free, instant, needs TA). If you wisely balance these three while maintaining stim boost and your HoTs, you're invincible. Clearly it's a case of easier said than done, but don't sell the class short because it's attached to the rogue kit. It can be a powerful healer (especially raid healer) in the right hands.
If I could add one thing to operatives and scoundrels, it would be a direct AoE heal that consumes TA. This would balance well with the other AoE heal (a HoT) that does not need TA.
Mercenary / Commando
Base Class: Bounty Hunter / Trooper
Healing Talent Tree: Bodyguard / Combat Medic
Closest WoW Equivalent: Shaman (reactive shield like earth shield) / Warlock (!) (mechanic similar to soul shards)
Equipment: Heavy Armor
Primary Stat: Aim
Unique Mechanic: "Combat Support"
Talent Trees: Mercenaries (and commandos) are, despite the heavy armor, play like a traditional caster. They have two ranged DPS trees, and one healing tree. If you like the idea of playing a mage in heavy armor that can heal, then this is the class for you.
Resource: Mercenaries use heat while Commandos use ammo. While the bars look a little different, and heat is generated in reverse, they function identically to energy. They have a limited cap (100 heat, 12 ammo) and variable regen. See my explanation under operatives for more information on energy, heat/ammo regen, and pacing.
Unique Mechanic: Combat Support is a stackable buff that increases your healing. It is generated by healing others with your free (no heat/ammo) heal, and with your cast heals when talented. Once the stack reaches 30, you can consume your stack of CS to vent heat (restore ammo) and give your abilities a temporary bonus effect (think warlock soul shards) for 10 seconds. Any abilities you use in these 10 seconds can get the bonus.
The regular heals are:
1. Rapid Shots: Instant, 0 heat, generates combat support (3 CS)
2. Rapid Scan: 2.5 cast (2.0 after talents) heal, 25 heat (generates 6 CS with talents)
3. Healing Scan: 1.5 cast, 16 heat, has cooldown
4. Kolto Missile: Instant AoE direct heal, 16 heat, has cooldown
5. Emergency Scan: Instant heal, 0 energy, has cooldown
Combat Support Bonuses:
1. Removes cooldown from healing scan
2. Gives small damage-reducing shield to kolto missile
Other Healer Abilities:
1. Power Surge: Removes cast-time from one ability (i.e. nature's swiftness)
2. Cure: Instant, Removes debuffs (can be talented to provide direct heal, too)
3. Kolto Shell: Shield, 10 charges, heals target when taking damage (i.e. earth shield)
4. Vent Heat: Vents heat in an emergency (same as gaining ammo for trooper)
5. Supercharged Gas: Consumes 30 stacks of CS to grant temporary bonuses for 10 seconds, vent heat/regen ammo, and increasing healing done.
Mercenaries and Commandos have a really visceral feel to their healing. You can really fortify the tank with a lot of heals: Kolto Shell, a HoT proc from your main heal, a damage reduction buff from your AoE, and easy access to an instant emergency heal just in case. Once you have your tank setup with these perks, you don't have to focus as much on him, affording you more time to use your "busy work" heal (rapid shots) on the others to generate CS.
It's vitally important you generate CS, too, since the CS buffs keep you going. You need it as a stable way to vent heat (regen ammo), it removes the cooldown from your cheaper heal, and it gives you a self-buff to your healing. When times get stressful and you don't have the CS buffs, you're stuck relying on Rapid Scan. That will oom you really quick, so you'll want to have Healing Scan off cooldown.
If you're good at triage and maintaining CS, it can be a relaxing healing style. You're constantly busy, so you'll start zoning out because you have nothing to do (or feel like you should be doing more...). If I could improve Mercenaries and Commandos in one way, it would be giving them a weak HoT that generates CS as it ticks. That would alleviate some of the busy work, which is admittedly the last thing you want to think about in a stressful (boss) situation.
Sith Sorceror / Jedi Sage
Base Class: Sith Inquisitor / Jedi Consular
Healing Talent Tree: Corruption / Seer
Closest WoW Equivalent: Discipline Priest (absorptive shields), Resto Shaman (similar mechanics to riptide)
Equipment: Light Armor
Primary Stat: Willpower
Unique Mechanic: "Force Bending"
Talent Trees: If you're looking for a very traditional caster experience, sorceror and sage are your best bet. Two trees are dedicated to spellcasting damage, and the third is focused on healing.
Resource: Sorcerors and sages use force as a resource. It feels a lot like mana, but it is limited. You cannot stack force via gear (it's always capped at 500/600 depending on talents). Unlike energy/heat/ammo, force does NOT regen at a variable pace. It regenerates at a constant 8 force/sec. This makes it safer to spam heals and dip lower in resources, since you're not permanently punished with a low regen.
To compensate, your heals are more expensive, so when you do have to spam, you will dip a lot lower than the other healers (ideally) will. You have some tools to help get you back on track. Consumption (i.e. life tap) can restore force at the cost of HP and regen speed. When talented, however, you can avoid the negative costs and keep the benefits by weaving innervate into your rotation.
Therefore, like the operative, sorcerors rely on a decent critical chance for their energy-regen talent to work; unlike operatives (who use cunning), they do not stack crit as well, making it more of a gamble. This is where Force Bending comes into play (see below).
Unique Mechanic: Resurgence is an instant heal + hot (like riptide) that alters the mechanics of your next heal (also like riptide). Resurgence is on a short cooldown (again, like riptide), so you can easily weave it into your rotation. The bonus effect caused by resurgence (called force weaving) only effects the next heal.
The regular heals are:
1. Dark Heal: 1.5 cast heal, 50 force
2. Dark Infusion: 3.0 (2.5 after talents) cast, 55 force
3. Innervate: Instant heal + 3 second channel, 40 force, has cooldown
4. Resurgence: Instant heal + hot, 30 force, has cooldown (talented to give bonus on next heal)
5. Revivification: 2.0 cast AoE direct heal (+HoT for people remaining in area), 100 force, has cooldown
Force Weaving (Resurgence) Bonuses:
1. Reduces force cost of Dark Heal by 50%
2. Reduces force cost of Revivification by 30%
3. Increases crit chance of Innervate by 25% (which in turn, crits allow you to use Consumption without taking a hit to regen/health)
4. Lowers cast-time of Dark Infusion by 1 seconds
Other Healer Abilities:
1. Reanimation: Combat rez
2. Purge: Instant, Removes debuffs (can be talented to provide direct heal, too)
3. Static Barrier: Absorption shield (i.e. Power Word: Shield)
4. Extrication: Lowers target's threat and pulls them to you (i.e. life grip)
5. Consumption: Instantly regenerates force at cost of life (like Life Tap), also lowers force regen rate momentarily
As you can see, sorcerors and sages have the most toys to play with. They also have the biggest library of heals, and with the force bending mechanic, they have the most versatility and dynamism in their playstyle. Without a doubt, the most effort has been put into polishing these classes, so it's not much of a surprise. I cannot foresee many situations where they would lack the appropriate heal to respond.
One trap to avoid is, do not think sorcerors are the easiest because they have the most tools. They still require planning and quick thinking like the other healers. If you play it blindly and do not use Force Bending to your advantage, you'll oom super quickly. Your AoE heal costs an outrageous amount without Force Bending, and its necessary to boost crit with innervate.
You sometimes need to think three steps ahead. If you're getting low on force, you first need to use resurgence to get force bending, then you need to use innervate to get the consumption buff, then you need to use consumption. That's several steps to just boost your force points. Try doing all of that during a boss and healing everyone else! As I said, it's not necessarily easy.
Leveling can be tricky since you must rely on "bare" (not innervate-modified) consumption. The unbuffed skill is best left to emergencies, and should not be used as a regular method to get more force. This will force you to learn some triage skills as you level, but just remember that it will get better and easier. (This is true for all the healers, actually.)