I don't really care for Choo or Shields & I'm not keeping a catcher. If you can't move these guys for someone else I'd keep Shields. He's a safer keep & as others mentioned, Choo should fall a little farther than the others.
Just because catchers are riskier than other position players doesn't mean never keep them, ever. I think people are forgetting Posey's minor league pedigree and MLB performance before he got hurt. He's got the age advantage, for one thing.
This is mainly a feature of me not liking Choo or Shields very much, but don't write off Posey; he's got the talent to be a top catcher. If I had to take one of them...