Sort of hard to say.
I honestly find ME1 to be a bit clunky after playing 2. Despite all the belly-aching about how they stripped out so much of the complexity, there definitely was an upshot to streamlining the game and it's hard to go back. That said, getting the chance to play through the whole Mass Effect experience with transfers between them (and eventually to 3) is probably worth it at some point.
Bioshock is an amazing game. A unique setting, a truly great story, and really memorable characters do a lot to recommend it. If you're looking for something new, that might be a better bet, and hold off on ME1 until later.
As for DLC, if you wanted to spend much less money right now, Lair of the Shadow Broker was, to me, probably better than all the rest of ME2. It's got all of the improved gameplay of ME2, but with the more fantastic locations and sense of mystery that ME1 had going for it.