In Bioshock, you'll get an extremely polished first person shooter that clocks in around 12 hours and offers little innovation in the gameplay department, but redeems itself with possibly the most interesting environment in gaming history with a great story that pulls you in. You'll find yourself spending a lot of time just peeking out windows to get a glimpse of your beautiful surroundings.
In Mass Effect, you'll get a slightly buggy 20-30 hour game that really plays how you want it to. Want a straight RPG? Play a biotic and run your battles through menus. Want a shooter? Go for a soldier and run 'n' gun the game. The dialogue system is absolutely stunning, as are some of the levels. The story is one of the best in video game history IMO.
You can't go wrong with either game but I would probably buy Mass Effect and give BioShock a rent. You don't want to miss either one of these games.
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