Wow, good question! Well let's think about this, if it were Vampires we would all, most likely, become immortal, right? Which might be kinda cool, however, still a little scary - think of the world population as a bunch of immortals that can only be killed by stakes through their hearts, Wow, that'd be nuts.
Zombies - now this would be pretty sweet (I mean if you weren't a zombie, that is). See Zombies would at least be an adrenaline filled hunt for as long as they stayed alive without completely decomposing. Now, this case would cause the world population to basically be wiped out except for the people who are not bitten by these flesh-eaters - thus, causing a rebuild of the world, which might not be all that bad of a thing.
Hmm, each has their interesting advantages - but since I'm a zombie nut - I'm gonna say that I would prefer zombies! I definitely don't want a million immortal George W. Bushs, or Al Qaeda after me as vampires. That being said, if zombies outbreak, everyone haul-up in a Fleet Farm (it's definitely the best place to be if this occurs)!