Well they both started out well. I mean Jackie's old kung-fu movies had some super stunts and Jet-Li's older ones (period movies) were really good. In the end I'd have to say that Jackie chan changed too much into making mordern movies while Jet-Li remained a little more true to the martial arts.
If I really had to choose between the two given their current list of movies I'd probably choose Jet Li simply because I'm a bigger fan of the period movies than of the modern ones.
On a parallel note, Donnie Yen kicks ass too. Iron Monkey had some of the best choreographed fight scenes in the business.
