This is actually a copy of a post from our guild forums. Lots of people asking this, and I think it’s not quite the question they actually want to ask.
My default answer to this is, as always, play what you want to play. If you base your game on other people’s needs, you’ll burn out. I’ve seen it from roleplaying on a MUSH all the way up through every MMO ever. Play what you want to play - let the role thing balance itself out.
Now, on the other hand, it’s possible that some of you asking “what Advanced Class should I pick?” are really asking “what’s the difference between my Advanced Classes?” That I can answer. (If you have experience with WoW, this link might help you decide, too.)
The Jedi Guardian advanced class wields one lightsaber and carries a force generator or shield generator. The Jedi Guardian is capable of tanking or damage-dealing.
The Jedi Sentinel advanced class wields two lightsabers. The Sentinel’s only role is damage-dealing, and you have a few options of how to do that - slow and steady, with damage over time, or with fast acrobatic weapon strikes.
The Jedi Shadow advanced class wields a double-bladed light saber and carries a force focus, force generator, or shield generator. The Shadow is capable of either tanking or damage-dealing; in the former role, it relies on damage mitigation, in the latter, it plays a lot like a traditional rogue class, although the Balance tree combines melee and ranged for a sort of ‘battlemage’ playstyle.
The Jedi Sage, near and dear to my heart, is a completely ranged playstyle - that lightsaber is basically for show (and deflecting ranged attacks sometimes), and your offhand item is a force focus. The Jedi Sage can heal, or it can do damage with its damage-over-time abilities and telekinetic attacks.
The Vanguard advanced class wields blaster rifles and a power generator or shield generator. The Vanguard class is capable of buffing up its shields and tanking, or getting in close for a unique melee/range mix playstyle of damage-dealing. (You get to knife people. How fun is that?)
The Commando advanced class wields those big freakin’ assault cannons you’ve probably seen many of them carrying, and also a power generator - I think. The Commando is capable of dealing damage - big surprise! - but they can also focus on healing. You literally get to shoot people back to good health.
The Gunslinger advanced class wields two blasters, and - much like the other dual-wielding AC, the Jedi Sentinel - the Gunslinger is there to deal damage from cover, and lots of it. The Gunslinger can specialize into better guns or better bombs, but one way or another, you’re dealing damage.
The Scoundrel advanced class wields a blaster pistol and uses a scattergun or shotgun as their “offhand” weapon, though you only see it during special attacks. The Scoundrel has two totally different playstyles - you can be a healer, and heal people from range, or you can deal damage - and the Scoundrel, unlike the Gunslinger, gets up close and dirty, shooting people in the back at point blank range with the Scattergun.
Lest you think I forgot about you, here’s a quick Republic-to-Empire class and AC primer.
Jedi Knight -> Sith Warrior
Jedi Guardian -> Sith Juggernaut
Jedi Sentinel -> Sith Marauder
Jedi Consular -> Sith Inquisitor
Jedi Sage -> Sith Sorcerer
Jedi Shadow -> Sith Assassin (Balance -> Madness)
Trooper -> Bounty Hunter
Vanguard -> Powertech (blaster rifle -> single blaster pistol)
Commando -> Mercenary (assault cannon + generator -> two blaster pistols)
Smuggler -> Imperial Agent
Gunslinger -> Sniper (two blaster pistols -> sniper rifle + power generator)
Scoundrel -> Operative (scattergun/shotgun -> knife)