Between those two choices, you should get Resident Evil 5 if you have another person to play it co-op with, but if you have to play solo, probably Dead Space. Like Gears of War and Army of Two, RE5 is really meant for two players, not one. Having an AI bot with you for an entire game is not my idea of good times.
But Bird is correct. Dead Rising is the best zombie game ever made. It's not even a close contest.