This is really a good question. Manziel does have some Favre-like traits. He could have a super high ceiling. But I wouldnt be shocked if he bombed either. Tannehill looks more predictable with his 6'4 225 pound frame, strong arm, smarts, and work ethic. I'd say Manziel is a risk reward guy. Tannehill would be stable. If I had a high pick, I'd take Manziel. It's not a Miami issue though.
I remember when I was a kid, someone asked me who was going to be better in the NFL between Sammie Smith and Emmitt Smith. I said Sammie. How did that work out for me? Humble pie. But, you know, that's how you learn how to project. Look at your mistakes and see where the thinking was off. Sammie was bigger and faster than Emmitt. I recall seeing Sammie go one way, turn around and house it the other direction at FSU. My focus on something like that was way off. Overall, as fans, we are hampered in projections because we can't talk to the kids or their college coaches, to get a sense of their toughness, brains, etc. We'd know a helluva lot more if we had that access. Those unseen aspects are a huge part of the process. It's not just their becnh, their shuttle, their 40 and other underwear aspects.