As the death metal vocalist I think I can safely share my opinion on rap vocals. There's actually a lot of similarity between death vocals and rap vocals. Death vocals, mainly monotone, with a ferocious changing of pitch between growling and screaming, but not really based too much on melody. Usually based on rhythmic timing.
Rap vocals on the other hand... usually pretty much the same thing, but without the growling or screaming. Monotone, almost like speaking in a rhythmic manner. Though I believe growls can express a lot more emotion than raping. Rap vocals stop being rap vocals when they introduce melody or auto-tune, with the exception of douchebags like Kanye West who put auto-tune on their monotone talking.
I'm obviously biased so I can't say one is better than the other, I'll just say they are fairly equal, at least on the surface. It takes a lot more vocal effort and control to be able to pull off a constant string of growling.