When your starting quarterback is a guy that's still relatively young, I always favor having a veteran backup. Besides the stability, they help act as a mentor/supplemental teacher to the quarterback coach.
So if it's not Anderson, I'd prefer we go free agent.
Preferably, you're gonna want a guy who's been a starter at least at some point in his career (what good is experience if it's all bench experience?) Plus you'll wanna pay attention to the systems they've been in (no WCO guys) and you absolutely don't want guys with a history of issues (so hell no on Vince Young).
There are a few guys to choose from in that list who could work.
Garrard would seem like a reasonable choice from what you said. He's a very good character guy from his days in Jax and even seeing him on hardknocks he is very.... "chill".. He would probably rub off on Cam really well.
While I see the logic in your stance, I kinda want a backup who can still win games if the unfortunate happens... Like Matt Moore/ Campbell?. They will probably be more pricey so I would be content with Garrard/Josh Johnson.