You're the one that brought up those names, not me. You said that right after you called him "exciting".
IMO, Manziel has a chance to be to spread quarterbacking what Manning was to pocket quarterbacking; a savant. Where Manning installed a playbook for his OC, Manziel may uninstall one. Manziel's tao understanding of space and broken plays is amazing. And, as you said, it's exciting. He could very well end up being the Jeff George of quarterbacking too.
I will never hold acting like every other 19yo in college against Jonny. Dude went out and risked brain damage and paralysis every Saturday, for some southerners amusement. Good for him for playing football at a high level while still getting to behave like other 19yo's in college. Really, good for him.
I'll always have a personal bias to dudes that flash that type of lifestyle as a middle finger to those who say you shouldn't. My gut tells me now that he's getting PAID to actually play, things will be different.
Mental shortcomings? You know him? Is there a tabloid magazine or something?