I think rap is better than rock, mainly because I like the sound of most rap (old school rap) and the lyrics are usually really great! More true when you find a rapper from the underground and has talent, like Immortal Technique. Rappers like him just rip 50 Cent to pieces. Also, what I've heard of Grand Master Flash is good too. NWA, old Ice Cube, Da Lench Mob really good groups/artists!
But,
Rock alone is better than rap, i do like the rock/rap combination though.