For roll-under mechanics, I have always preferred "Higher is better, until you're too high." If Sarah has a 15 and Bob has a 13, Sarah MIGHT succeed, but less than Bob (i.e. she rolls a 7, he rolls a 10), and it's easy to tell; you don't have to worry about "margin of miss" rules or the like, just straight number comparison. But since Sarah can roll a 14 or 15 and succeed, there's a range where she can succeed and Bob simply can't.

It also meshes with the Psionics system, which is the only explicit choice on the roll-under mechanics.