Slow zombies for sure.

As with others here, I do think they make more sense than fast zombies. But, even without that, slow zombies are better for stories. Anything you want to do with fast zombies there's almost certainly some other setting-appropriate monster that can be subbed in. Shamblers, on the other hand, are one of the only traditional things out there that can fill the "slow moving horde of death that is more a force of nature than a bunch of monsters" role. Even just a couple shamblers showing up is almost like a warning of a storm approaching, whereas a couple fast zombies (from a story beat PoV) is not that different than a few orcs showing up as scouts of the orc army.