I think the main thing is that both supporting casts are tailored to the respective individual. Lebron has a group of spot-up shooters who float around the perimeter and wait for him to go one-on-one to create a jump shot opportunity. Kobe has a group of players that compliment his skills in the triangle offensive. Neither player would be as successful on the other team. Kobe at his age doesn't drive the lane enough to create the open looks Lebron does and Lebron would be lost with the sophistication of the triangle and his poor jump-shooting would be a problem.
Lebron probably passed Kobe as an individual force last year, but I still think Kobe's skill set, which includes mental attributes, is more appropriate for a championship team. For all his brilliance as a one-on-one player, Lebron's style of going one-on-five against the best teams is always going to have a hard time beating a squad that can match Cleveland for talent and runs a balanced team offensive, like the Lakers, Celtics, Magic, and possibly the Spurs.