Both excellent, excellent musicians, but my vote goes with Jimi Hendrix. Leaving my personal preferences and reasons out of it, Clapton was more of a traditional guitar player, until Hendrix cam and blew him out of the water. He practically reinvented the electric guitar, the guitar was a 'part' of him, like and extension of his body. He used new techniques, and did things with a guitar that people didn't even know existed before he did it. He changed guitar playing, full stop.