If I had to choose, I'd go with Half-Life 2. It came first, and is therefore more nostalgic to me, and it has more variation in the gameplay. In Bioshock, you're always under the sea, fighting splicers and Big Dadddys. HL2 mixes it up with spooky zombie bits, epic strider battles, giant bug sheparding, and the awesome part at the end.
Still, I love them both.