Kickboxer hands down just from the fact that they have more tools at their disposal. No contest IMO.
I fear not the man that practices 1000 kicks once, I fear the man that practices one kick a 1000 times. ~bruce lee
I wanna say the kickboxer generally. But I think it depends on the type of fight, the fighter and a lot of other things. Boxer is probably going to have better hands and movement, but boxing has a lot of holes when defending these "new" strikes they'd face against a kickboxer.
I would love to see klitschko vs bonjasky in a modified K1 bout, although im not sure what the rules would be.