Sometimes depends on what kind of protagonist we're talking about. If monster then appearance actually does make a difference. The more believable the design, the more drawn in you'll be and more disturbing the movie. If a hobo with over sized canines, you're not going to be as drawn in because it's harder to take such a silly appearance seriously.
It's
apples and
oranges.
Personal preferences in slasher or human in general killers though I really don't know. I like the tall, dark, silent types like Micheal Myers and Jason Voorhees. But I also like the sophisticated, silk tongue of Hannibal Lecter... And cannibals in general intrigue me. So do lunatics. Crazies are fun cause never know to expect.