IMO: Slow. With an explanation.

Depends on if the universe sticks to the observational rules the characters witness.

Slow zombies will be there for awhile. Not that scary, but always a background threat, for potentially months, maybe even a year. Slow also means far more quiet. No big footsteps, a bit more stealthy, harder to hear coming unless in large groups.

Fast zombies should in theory be easier to hear coming. They're also usually not zombies. It was a virus in the 28 days/weeks movies. They'll also burn their reserves out faster, unless they can find food. So, many at first, but there will be a lot less of them each and every day as they burn out and starve.

But the whole adenosine triphosphate issue would make neither of them actually be able to move if they're going with a science angle.

Not to mention that most zombie survival movies/comics/whatever don't have people searching for potable water very often. A bigger issue than the zombies.

So, slow if you're looking for more of a character driven story with heart and emotional connections, a sort of family bonding through shared experiences.

Or fast zombies for a more visceral action oriented thing like a movie where every moment to moment is won by being not killed by them until you find a clever macguffin solution.

Personally, I'd go with slow. More options, more zombies. They'll in theory last longer, always a threat, even a minor one. Going out from your safe spot to look through a store could mean the end of you if you are not careful. Even months after the incident that brought them about. If you use all the resources in one area, you can move to another and start from scratch with what you have and repeat what you've learned. Make a new safe zone, find food and water, etc.