I can see the comparisons from the two QBs. I will say that from a talent evaluation perspective, Leaf was much better then Manziel. Heck, he was much better then Manning, just from a pure talent perspective. The thing with Leaf, and don't get me wrong, he did plenty of stuff to screw himself but the thing about him was that he had some bad circumstance to deal with as well. Remember that San Diego traded up to be able to take Leaf. They traded away a whole lot to get that second pick. A couple of 1st rd picks, some mid level high picks and Eric Metcalf, plus, they signed him to a record deal. Add that to the fact that Kevin Gilbride was his Coach and that guy's offense is terrible. He is to offense what Ryan was to defense. Very complicated and very hard to execute. Heck, even Manning and Warner had trouble with his offense when he was in NY. They finally kicked him to the curb. Leaf was not exactly a road scholar so he had some major issues with that offense. Just before his second year, he injured his shoulder and in those days, you really couldn't fix a torn labrum. He was never the same. However, he was not completely done, he just couldn't throw the ball like he did before the injury. He came back in 2000 and was never really the same. He injured his wrist and that was pretty much it. He failed the physical with us and that was the end of Leaf.
I often wonder what he might have developed into had he been able to avoid the injuries. I don't really think Manziel is the same situation as Leaf but I can see why some might, in light of the behavioral issues each experienced.
If I were comparing him to a QB, it might Kerry Collins out of Penn State.