I don't know enough about how Jake Locker is currently performing to enter that discussion. I did watch him do well in college, especially with his legs. I know he's had problems staying healthy in the NFL though. Never a good sign.
BUT, Mettenberger is a legitimate pocket-passing QB at the next level. I know he's just a rookie, but from his JR year to his SR year at LSU under Cam Cameron teaching him, he made huge strides in just that one year. He's very coachable, has a very good arm, and is your proto-typical sized pocket-passing QB.
He doesn't have any scrambling ability, but if you give him an O-line that can block and give him time to throw, he will pick apart defenses as he learns the NFL game. Still might be too early to tell with Mettenberger, but if he puts on some more weight (muscle) and can stay healthy (he's still skinny/lanky), he will end up being a good QB, IMO. I liken him to a Joe Flacco in just build and such.
I'm a 20 year LSU season ticket holder so I may be a bit biased here (lol), but I just want to say his JR year he was a deer in headlights when the rush came. Moved awkwardly inside the pocket at the same time. Like he couldn't get his feet under him and his arms just flailing around.. the next year our O-line got healthy and gave him time, and under one year with Cam Cameron he flourished. If I was playing with Titans in my CFM I would certainly groom him to take the reigns over Locker. But hey, that's just my opinion.
I hope he does well with the Titans. I may take him in my CFM with the Rams actually.