First off, I just want to say that Jet-Li is FREAKIN' CAT- QUICK! I'm a huge fan and I LOVE his fighting style, as well!
The thing is, Bruce Lee is my 'hands-down' pick.
Bruce Lee was a Jeet Kun Do (Martial Arts style) founder AND teacher. Every day he trained himself to be peerless. It has been documented that he would go around the world searching for other martial arts techniques that were popular and would challenge the Dojo. It was also proven that Jeet Kun Do is one of the top systems, if not THE best technique in the world and a hybrid style he developed from studying multiple teachings from various techniques. Bruce Lee also studied and did research on the human body to find its limit of movement and weakness points. He would then test those in competition. We've seen those results.
Jet Li studied Kung Fu and the the Wu Shu practices and just perfected what he was taught. He is definitely a great movie stunt fighter hands down. Again, no disrespect whatsoever to Jet Li. I'm one of his biggest fans. I'm not saying that Bruce Lee is God or taking anything away from Jet Li. It's just my opinion. Any pick would be a good one.
The bottom line is that if they did fight, WE'D all win. =)