I prefer Medic for personal reasons. I simply hate the Soothe noisy, zero-information, spammy and uncustomizable animation of this spell that it means I will never heal on my Esper, hence losing half the class right off the bat.
Esper is easier to casually solo with than Medic, especially tougher named quest mobs or weak prime mobs, since you have the option of using its high mitigation and high lifesteal build. It is also built from the ground up with a rotation skill system rather than priority system like many of the other classes. It has multiple off-GCD skills, which also happen to be resource builders. This means you can spam those skills while using your main attack then deal out finishers rapidly. All these combine to make it much easier to learn and quest than Medic. Despite this, many find the Esper far too static and unwieldy for their taste. This is mostly due to the untoggle-able and stationary special power Innate spell. Hence, why it is the least popular class.
Medic, by contrast, has almost zero off-GCD skills, even utilities like cleanse and worst of all not even on ground, targetted aoes and charged abilities. This means from an efficiency perspective you are paying up to three times more than on other classes per skill, i.e. you have to first click it, then target it, then click it again to activate it, then on top of that the GCD hits and you have to wait it out before you can do the next ability. It has little in the way of direct mitigation and almost no lifesteal compared to an Esper. What it does have applies almost exclusively to shields or requires using active skills (Protection Probes), taking up space in your limited skill slots, hence compromising damage. It also has some Priority abilities and AMPs that require optimal resource management. All the above combine to make it harder to quest on a Medic than Esper.
Scientist is ultimately more useful to the group than Settler because you can use it to Summon players. However, it is also by far the hardest Path to take and the Summon ability is on a long cooldown (you are no Warlock!). Personally, I prefer Settler because it has the best mix of utility without wanting to blow your brains out and its great for new players because while levelling up you really feel you are helping out other people by being able to create very useful stations at Settler points.