I played bioshock but not mass effect.
I really enjoyed bioshock, but it didnt have that 'stay up all night to finish this level' quality. Dont get me wrong, it is an awesome game. While its spiritual predecessor, System shock 2 is in my top 3 PC games of all time, i thought bioshock was very well done, but maybe not quite up to the level of the classic. Bioshock lacks the RPG system in SS2, but the complex storyline in bioshock more than makes up for that IMHO. It may be sacrilege to say so, but i had more 'piss your pants' moments in bioshock than in SS2. (Which is saying something cause there were quite a few in the classic).
I have mass effect on the way. So to the OP i would say play both. Get the cheaper game first, then trade it for the second.