If you toss scientific facts aside (because let's face it, Disney movies are NOT known for their scientific accuracy

), it becomes rather clear that Simba is healthier than Scar.
If one takes a lok a Scar, his mane appeares thin and slightly dishevelled, and he's lean almost to the point of unhealthy. He say's himself that he was not born with brute strength. He simply does not looks he cares for himself very well.
Simba, on the other hand, looks much better. He also possesses a small build (at least far smaller than Mufasa's), but not to the extent that Scar does.
Toss aside science, and look at simple looks, and it seems clear that Simba looks far healthier. At the very least, while Scar is "thin", Simba could be classed more as "athletic".