Division I - 5A is tougher. Division II isn't better because it has a higher number... just is the rich private school league.
I would assume most of the Division II schools would fit in nicely in 4A... the very, very elite in 5A and some of the crappier football schools like Baylor in 3A.
Do you know why they split into two Divisions? It wasn't the public schools begging for a seperation, I'll tell you that much.