I was actually a bit perplexed by this dilemma, as they couldn't be more different games. Maybe which one to buy first, but not either or.
Anyways, Dead Rising is going to be out in like days, looking out at you from behind your local EBGames representative, saying "But, don't you like killing zombies? And watching desperate humans shoot other desperate humans over cans of soup? And clowns juggling chainsaws?"
And a voice in your head will say "But, but the reviews said it was a short game."
And Dead Rising will reply, "You could get hit by a bus tomorrow. Who cares about the length of a game."
And you will have a very important decision to make.