The difference is Leaf probably could've been good if he wasn't an idiot. Manziel could have Peyton Manning's work ethic and I doubt it would move the needle a ton for him.
I never understood the infatuation with him. He is a short small guy that runs around. He has always showed himself to care more about getting pussy than football. If you take a qb in the 1st round, you need an instant grown up ( fair or unfair) not a kid that from what I hear never needs to work in his life and still be set up well.