Neither. Reanimated cannibalistic corpses, whether fast or slow, pose a serious threat to the delicate fabric of society. Therefore they should be expunged mercilessly from all cities (with the exception of San Francisco), and we should not practice zombie discrimination.
It depends on the movie, really. What are they looking to get out of their zombies and what kind of movie they want.
If you want more action packed go fast.
If you want suspense go slow/realistic.
I'm an old school snob, so zombies must be slow and shamble like a drunken hobo. Shaun of the Dead's confused creatures, not I am Legend/World War Z rabies induced athletes.
Slow zombies, it's the feeling of being trapped and watching in horror as the horde closes around you, you've run and run and run, only to find yourself at a dead-end.
The zombies shamble towards you, "erhaps due to rigor mortis? No, no time to think about that... there has to be a way out, it'll take 'em a minute to get me" you think... "Can I fight my way out? No... not enough ammo... this blade will take too long..." they draw near... you see a door!
...
"It's locked"
As the apparent wall of zombies come closer, seconds away, you see a window open... you reach... you're late...
You're one of them, now.
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It's why Resident Evil worked so well in the early days, the suspense of seeing such a painfully slow foe, but unable to find a way out!
No, they need to be extremely slow, or not moving at all. Otherwise, the single-frame-teleport-right-behind-you trick will be completely wasted. They should also be covered in talcum powder and wearing capes that can be drawn in front of their faces to conceal the actor's identity.
