Alright, first of all, there’s a pretty basic taxonomic issue we have to address before we discuss zombies. Fundamentally, there is not such thing as a fast zombie. If there is a mindless, murderous, decaying person running after you, you’re not dealing with a zombie, you’re dealing with a person who has been infected with the Rage Virus. A zombie is a corpse, reanimated by magic or radiation, which sluggishly walks among us, either performing labor for a sorcerer, or, more commonly, seeking to devour the flesh of the living. The infected are simply people suffering the final stage of a deadly, contagious illness which causes rapid neural decay and involuntary outbursts of violence.
Now with respect to the question of whether zombies or the infected are scarier, I think it really depends on how we conceive the issue. Certainly, on a personal level, a running “zombie” is more dangerous. If you happen to encounter an infected person on a burned-out, post-apocalyptic street, you’re much more likely to get caught and disemboweled. But taking a broader view, true zombies are much more serious threat to the human species.
Zombies eat the flesh of the living, which means they can form a self-sustaining population (at least until humans are close to being wiped out entirely). The infected, however, don’t eat their victims. Combine that with a higher caloric demand due to all that running, and basic chemistry demonstrates that starvation is bound set in very quickly among the infected. In the case of an outbreak of Rage, all you need to do is hunker down in a safe space and avoid getting involved in a Hobbesian war of all against all with the other survivors for a few weaks, and you can walk out into a virus-free world. So while it might be easier to dodge and even pick off a few of the walking dead, you can’t expect to wait them out. The authentic zombie realizes a fundamental truth of biology: slow and steady dominates the surface of the Earth.