Chris Rock gets a LOT deeper with his jokes in my opinion. As was said he touches on more diverse topics, and I think his standup routines flow a lot better from joke to joke. It sounds natural. Katt's great, but he's got a more limited range (I don't even mean he just talks about the pimp angle...he's just not as versatile as Chris Rock). That, and as was said, he relies more on expression and physicality than Chris Rock does...Chris Rock is sort of a funny-looking nigga anyway, so he doesn't have to use that as much.
And Chris Rock's movie appearances > Katt Williams'. Easily. Katt's got time on his side though, so I think he'll make the gap closer