Well lets assume there are no rules. A kickboxer would want to keep their distance to use their more highly trained kicks and punches and a muay thai boxer would want to grapple to use their knees and elbows. As long as the kickboxer was decent enough to keep out of the grapple then they would have a good chance of winning. Not taking anything away from them but a big reason that Thai's tend to win alot of their fights in Thailand is the rule system there that rewards elbows and knees. Muay thai is definately a sport, the national sport of Thailand. Either can be a way of life or a hobby. If I were betting my money would be on the muay thai guy though...but then I do muay thai.
