I have never played Bioshock but I have never heard anything bad being said about it. I am for Assassins Creed all the way I have played and loved each installment and I am anxiously awaiting for the Assassins Creed Revelations to come out. It really depends
on your taste of games to determine which one of the two games is better.
To be honest I personally didn't really enjoy either (or what I played of Bioshock anyway...). But I think my disliking of Bioshock may have been a personal taste and seeing as a lot of people love it I might as well recommend that