I could discuss zombies for hours! But I'll refrain. I actually like them both, fast and slow. In my opinion there are no bad zombie movies (once you become a b movie fanatic, theres no going back). I read a rumor on io9.com that there is a zombie television show in the works based off of romeros work, and to settle the question for them, they are going to have fresh zombies (recently turned) able to move quickly, and decomposed zombies shamble. Makes sense to me, the more muscle/tendon/flesh/limbs/etc are missing, the slower they'll go.