Which spec would you guys recommend for best efficiency? I've heard fire can be troublesome with the aoe in mists. Is frost more efficient? Hopefully by the time i hit 90 the state of end game specs will be settled.
Frost is easily best for leveling, really good reliable burst on demand (not depending on luck like fire) and without need to setup damage like arcane. Then also good aoe burst every min if pulling lot of mobs. Multiple freezes, slows etc.
Frost has always been and will always be the most efficient leveling spec. It's also far too easy if you are already familiar with how casters work and are interested in a challenge. I know a Mage who leveled as Frost without ever knowing that her Water Elemental could Freeze a target at range. I knew another Frost Mage who spammed FFB instead of Frostbolt because they assumed it did more damage. It's not that Frost won't get you to 90. It's that Frost will be so easy that, after you get there, you might not (though, you may) understand what casters typically have to do to survive. This is the same drawback of Hunters leveling Beastmaster because they rarely need to use class mechanics such as Deterrence of Disengage.
Arcane is an interesting challenge, but a difficult one. Arcane has the potential to be an extremely fast questing spec. It has an initial ramp up time, but after that you can plow through an entire cave of enemies with proper mana management. The higher your mana, the harder your blasts, so you'd typically end up kiting while purposely stuttering your Arcane Blast casts. It's a very awkward playstyle, I'll admit. But it teaches you how to manage your mana and how to kite. The goal is to never reach 100% mana, but also to never drop below 90% mana (it's actually to never cast below 90% mana, in case my wording is an oversimplification). So you use Arcane Blast to drop mana, Arcane Missiles to gain mana, and likely Scorch to give you a chance at more missiles (though I prefer the challenge of fighting without it, you will probably die less often with it). Arcane is also capable of Shatter combos, but hardly (Arcane Blast + Fire Blast, not Barrage since you want to hold onto your stacks).
Fire isn't necessarily a challenge as it is a learning tool. Fire, like Frost, is capable of fight multiple enemies at once by kiting with AoE snares (Cone of Cold or Blizzard) and disorients (Dragon's Breath). And with the short CD on Combustion, you can use and spread it very frequently (and this is encouraged). Scorch becomes a necessity for questing as Fire. Your only freezes are Frost Nova and your Tier 45 talent (and Glyph of Ice Block, if you so choose). Scorch gives you much-needed mobility and can be useful on Shatter combos (Scorch + instant Pyro if you have it, Scorch + Inferno Blast if not) since freezes don't last through much damage and you'll likely have DoTs ticking. Fire teaches you to manage and keep track of your DoTs (for AoE, it's more potent to spread Pyroblasts, Ignites, and Living Bombs than it is to spam Arcane Explosion), kite using the tools of your class, and take charge of quest zones with good AoE and smart movement (i.e. no face-pulls).
Frost is by far the simplest. You can sit still and check your surroundings as you fight. You can plan your next move while completing your current move. You can use your pet to help speed up quests (eg. pull mobs away from a quest item on the ground) and do amazing amounts of AoE damage to large packs with Frozen Orb and a few well-timed freezes. You can even use Scorch and develop a more fire-like playstyle (but it's kind of wasteful). And if you really take it seriously, you learn to manage your Frostbolt stacks, plan the use of your cooldowns before they are available, position your character so that you have time to cast without taking damage, utilize your damage mitigation, use lots of CC (sheep, ranged pet freeze, deep freeze on a non-target, Ring of Frost on cooldown, etc.), and generally gain all of the skills that are necessary to become a master at PvP. But if PvP is not your goal, the chances of you "taking it seriously" are slim and the spec does nothing to force you into using any mechanics beyond Shatter combos and Deep Freezes.