As a fan of both rap and rock, saying one is "stupider" is well, stupid. They are both great, and there are rappers who suck and there are rappers that are great (some of the ones that are the most offensive I like the most). Because I am a punk at heart, I tend to like rap that is more grimey and offensive (rap is just black "punk" if you think about it), but i also love old Young MC (Pick up the Pace or Gont more Rhymes anyone!???).

Rap is driven by vocals (which is why it is so popular, most casual listeners of music care most about vocals and never really listen to the actual music, if you listen to mainstream rock or pop or hip hop you will notice that the vocals are juiced up and that the actual music is pretty bland) but lots of rock movements use vocals as background instumentation, sometimes barely audible and muddled. Shoe-gaze is one of the best examples of this, Listen to My Bloody Valentine and you will not understand a single lyric, but you know exactly what they are trying to say... good stuff.

Rock isn't always about the vocals, Rap is.