Both have their place. I don't think slow works in film, unless it is filmed specifically to mimic comic panels with small vignettes timeskipping to capture the relentless and infinite hordes. Because there is never immediacy to the threat.

I liked the 28 days zombies because it wasn't just the speed but also the 1 drop of blood and 10 seconds to turn that kept the tension permanent. At least until they had to throw in the obligatory "White man is the real monster" 2nd half.