I rolled an IA and I really both game play and the storyline, but I'm curious:

 

Which class and advanced class is the most different in terms of gameplay, and storyline?

probably sorcerers, if you're an operative; or assassins, if you're a sniper

 

for gameplay, you can probably guess why

for story, inquisitors seem more focused on meeting their own goals, and ignoring the empire's goals. Agents, on the other hand, are stuck doing the empire's dirty work; as you probably already know, the personal feelings/goals of sith have a tendency to get in the way of agents trying to help the empire.

That makes sense.

 

Here's a related question for ya'll:

 

I'm a sniper, which makes me pure damage, for any group activities (and even just running around with a companion) I usually just hang out at the back of the group and focus on the highest level enemy in a mob. Which is more different from damage-tanking or healing?

healing. tanks still kill regularly, and focus on combat with mobs. Healers, on the other hand, leave the actual fighting to companions/other players and don't usually attack during a tough fight.