Well, I'd personally go with Tetris. I like some retro games, but I just don't appreciate Centipede as much as I do Tetris. I think Tetris has some different game modes with special effects to spice up the game a little. Hmm... Maybe I should finally get Tetris... (I've got Tris, but i don't play it much)
Oh wow that's a toughie... Both are fantastically done retro ports. Easily the best 2 on the App Store Which game do you prefer? Are you into the retro-style gaming?
When I faced this dilemma Tetris was still full price so I got Centipede and haven't looked back since. With 3 levels of difficulty on each of the 3 modes (Classic - Arcade - Ultra) there's plenty of play value here.