OP, I have to recommend both Bioshock and Mass Effect. If it's a choice between the two, Bioshock but that's personal.
Mass Effect is a fantastic game in its own right and Bioware definitely draws upon its experience with KOTOR in creating it; almost to a suspect degree. The universe is definitely interesting and the supporting cast feel like real people. However, I say to a suspect degree because it feels a little too much like Star Wars. Biotics may as well be The Force and Spectres may as well be Jedi. A lot of it feels like KOTOR, moral choices, talking to your teammates, walls of dialogue (all of it very, very well acted and the responses have a great amount of depth), and this isn't necessarily bad. It's a step away from the Star Wars franchise for Bioware which is more than I can say for Lucasarts. Is it worth a play? Considering it's dirt cheap at any local Gamestop, definitely. Combat is fun, frantic, and relies more on strategy and using what's on hand rather than running in with the trigger taped down.
Now for Bioshock...get it. It's a wonderful experience. Bioshock is hauntingly atmospheric. The music ties in wonderfully to the dark and macabre creatures that lurk about each corner. Each enemy you fight wails on about how they're sorry, how they don't mean it; some even quote passages from the Bible in seeking forgiveness. It gives a feeling of utter depravity caused by one falling into the deepest forms of hedonism. Some enemies, on the other hand, revel in the destruction they're causing and will flail at you while telling you so.
This ties in with the story of a Utopian city that falls into despair at its own greedy hands. While a little preachy (CAPITALISM BAD RAWR) the story is wonderfully written and the twist toward the end leaves you with a sense of both satisfaction and anger. The final boss, while being absolutely easy, is actually someone you want to defeat. I would go on but I'm hesitant to give any spoilers.
As far as combat goes all options are open. The game functions on a weapon wheel so you have everything available to you for every situation. It's an absolute blast to lure a group of enemies into traps that you've set and watching them all crumple behind you. The amount of options you have in combat is staggering and you likely won't dry them out in a single play through. A lot of this is undermined though by the fact that the game just is not challenging. Rather than using checkpoints, you're transported to reviving chambers and all your progress is kept. In theory, you could smack a Big Daddy once with a wrench, wait to die, and repeat until it's dead. Despite this, Bioshock is something that simply cannot be passed up.
~Taz