Definitely slow. The greatest aspect of the traditional zombie is that alone, it is almost comical - it's stupid, it falls over, it's easily incapacitated (but not necessarily defeated). However, complacency will bring out the best horror aspect in the traditional zombie - overwhelming numbers. Eventually they swarm and consume everything. An unstoppable force, an army that marches without end or fatigue. You develop a sense of claustrophobia as they slowly but surely crawl in to consume everything.
The problem i have with contemporary fast zombies is that it makes them more 'human'. This takes zombies out of the uncanny valley. They have human qualities. They move like they are alive. They may grunt jarbled words or phrases. They appear almost human, but not quite. This no longer makes them appear supernatural or unstoppable. Humans can bleed, humans can die. If i wanted to face off against humans, i'd play a terrorist game or i'd watch a CSI flick, if you can get what i mean.