Rumble is pretty strong, but he lacks mobility compared to Lee Sin. However, as soon as people begin splitpushing waves and racking up huge items, Lee starts to lose his advantage somewhat, and starts to rely more on his utility with his movement, shield and ult rather than his actual damage.
If you like to play things close to the chest, can hit skillshots, and are very tactical about how you approach fights, Lee is pretty good for you. If you like to mash out in people's faces and destroy teams with damage, Rumble is a better pick.