I like the fast infected. For example look at Dead Island. It had both slow and fast zeds. Slow zeds where very easy to deal with and where only a problem in big numbers (10+). Infected on the other hand could be a threat alone. That game couldn't have worked if it was just slow walkers, it needed the infected and the specials to keep the gmae from getting dull. Walkers are only a threat if they are in massive numbers and if ammo was very rare. Even if with headshot only kills every person has the ammo to take out or kite an enitre town in Day Z. I vote for a mix. Have the zeds that see a play in the distance (150m?) start to shuffle/limp/hop to the players last known location, and any sound/sight would update that location. Once a zed gets sight on a player within say 50m (less?) then they run for a kill.