I was considering the BioShock 1 + 2 pack as well. I've played both, but don't have BioShock 2 on PC. Then I checked my library and discovered that apparently I bought that during the Steam Autumn sale. I'm an idiot.
Anyway, both BioShocks are very good. The second doesn't have quite as interesting a story, but I like the actual gameplay better - particularly the bits where you get to set up trap zones to defend Little Sisters. Lots of shooters give you mines, autoguns, etc. but rarely do you get to use them the way you do in BioShock 2.