I think it's very hard to tell how good a fighter an actor is by watching him in a movie. The movies are designed to make them look good. I'm not saying that they don't have any real skills; I'm sure they do. But if you watched them spar, I doubt you'd see all those fancy kicks, especially if they were sparring against someone who really challenged them. They are great stunt men, who can pull it off on screen. This is challenging all by itself. Just try to look that good on video yourself. Even if you have some good chops, making it look good on film is a whole other skill set. And that is what they are primarily good at because that's their bread and butter.
To the last comments, Jet was 6 time national champion of China, do not know Tony Jaa's actual martial arts background, anybody? Through the years, in order to be more realistic, I know Jet has trained under traditional internal and external masters to understand the true fighting nature of the CMA. I think both are incredible, but a young Jet v. Tony Jaa would be very interesting!
Jet Li is from Beijing. A Taiji instructor who is from China and studied internal and external MA in the university system there once explained to me that the 'National' competitions are team events between different regions, but Beijing is the capitol and the base of the Communist Party. For Beijing to lose would make the capitol and party lose face. Beijing never loses, ever. So whether or not Jet is a good fighter I can't say, but being national champion doesn't really provide evidence either way. Now the fact that he was competing period, and that he was probably the best fighter on the Beijing team might.
Who wins the fight? Whoever's name appears first in the credits.
Having said that, I'm a greater admirer of Tony Jaa. Don't get me wrong, I enjoy Jett's work. I just never enjoyed most kung fu on film, it seems to rob it of it's viceral impact leaving it to look just pretty a fluttery.