A mix, they have a different feel. Slow zombies (in large groups) are about that overwhelming feeling of being up against a brick wall... Like just get out of there and keep going. Fast zombies are more about the immediate "kill or be killed" thrill / adrenaline.
Same goes for games. I enjoyed Days Gone's fast hordes but its not... Scary. Its action movie stuff. Near the beginning it's "get to your bike and get the hell out of there"... Towards the end its "I'm stocked on napalm and heavy machine gun ammo so let's fkkn do this".
On the other hand, years ago I played "State of Decay" (which is more like a... Zombie simulator? More about scavenging and building a stronghold than storyline) which in original form is fast zombies, and wasn't very fun, but then I played through it with a mod that made the zombies unable to run but increased the NUMBER of zombies and it transformed it into an amazing game. Trying to scavenge some food or supplies from a house with a large group of zombies heading down the street... A slow inevitable march... And you're searching stuff like "ffs come ooon!!" then escaping out the back window at the last minute.
A *lot* scarier than 2 or 3 running zombies bursting in without warning.
On the third... Hand... There was a part of The Last Of Us where I was being charged by a stream of runners and ran out of ammo in seconds and was just in a corner flailing wildly with a plank of wood. It was one of the most brutal and terrifying things I've experienced in any game
.