Bart has had to face tougher opponents; Nelson; many occaisions, Jimbo, Gulf and Kerny; multiple occaisions + all at once most of the time, choked multiple times by Homer, ravaged by a wolf. Thats a very respectable list of tough opponents.
Cartman on the other hand, although has had his share of battles, has not faced the opposistion that Bart has.
Then we move onto speed, agility, age and experience factors, raw power, the ability to take a punch. All but the punching power I'd give to Bart, Cartman defenatly has the size advantage in this one and only give him a punchers chance at catching Bart with a KO.