Probably Jackie. Jet Li is in great shape and does modern Wu Shu very well, but modern Wu Shu is not meant for fighting. It's meant as show only.
Jackie, on the other hand, has actual experience in traditional chinese martial arts and in his youth was somewhat of a trouble maker. He also had to deal with thugs on account of gambling problems and with the triads because he broke a contract with a film producer. I believe we can safely assume he was in a fight or two in that time.
So, Jet may win on account of his better conditioning, and if a fight was scheduled he would probably learn some real MMA influenced fighting pretty quickly. But experience and background speak for Jackie, even though he is 53 now.
In conclusion, I believe I put way to much thought into this post and should consider getting a life. Thanks for listening.