From a martial arts perspective and as a fan I have to go for Jet Li... he simply is a better martial artist and I love a lot of his films like Fist of Legend and Warlords. My only real issue is that the English films he does tend to be rather cheesy, War, Cradle 2 Grave, Unleashed and The Expendables are all a bit cringeworthy at times but the action is still good. I also love Jackie Chan but his fight scenes while good aren't the same and often are made into more humour his older films were a bit more action and like many people I do love Rush Hour 1 and 2. When it comes down to it I simply recommend watching Warlords as it's amazingly well acted and all the fight scenes are fantastic.
Both are outstanding in their own right. Jackie throws in a lot of acrobatic movements in he display mixed with a bit of comedy. Jet Li with his serious and direct approach would indicate he could be much more serious and more dangerous
Jackie Chan. Before Jackie was seen as a funny guy he was whooping people's ass. Jet Li to me is more known as the guy in movie that really don't have much to do with Martial Arts, but he uses Martial Arts.