This is somewhat of a myth. If by that you mean Alabama, then yes that is the best competition. But the rest of the SEC is not that great defensively outside of Florida and LSU. There are some good defenses, but the SEC carries that reputation because of the 2-3 top tier teams and media hype. The Big Ten has 4 teams in the top 11 in total defense and the ACC has 3. The SEC has 2. Yet you never hear the hype about those defenses. Anyway....
Bridgewater takes better care of the ball than Manziel. Manziel has great improvisation, but running around and chucking the ball up to 1 WR and 3 DB's rarely works in the NFL. I've also seen a lot of QB's in this system put up insane numbers and never do a thing in the NFL. Manziel will have to learn a pro system with pro reads and progressions. That takes time. The NFL doesn't have any one-read then scramble offenses. Bridgewater has a huge head-up on him as far as experience in a pro-style offense goes.
Also, the majority of Manziel's effectiveness is based on his play out of the pocket. Just look at what NFL teams have done to Robert Griffin this year. They keep him in the pocket and make him beat them with his arm. They are dedicated to limiting his dual-threat ability. He's also been injured 4-5 times in a season and a half. Manziel is tough, but if he runs around like that in the NFL he won't last 3 seasons. His improv style won't be nearly that effective in the NFL and he won't have a guy like Mike Evans to bail him out. Evans is one of the best players in college football IMO. He is just an outright mismatch on 99% of the DB's he goes up against.
Bridgewater stands in the pocket and destroys everyone he plays against. He ripped apart a top 5 defense last year in Florida and rookie Raven Safety Matt Elam said he was the best QB they faced all season (UF played against Manziel, Tyler Bray, Aaron Murray, and E.J. Manuel). He is mobile enough to escape pressure and keep plays alive, but he's mobile like Andrew Luck not Robert Griffin. He's a natural pocket passer despite his athleticism.