I think "neither" may be as likely from management, given the part of me that expects Shields to be asking big money. But I think there is a possibility that the Packers will shell out big money for Shields. I think the chances are slim and none that they will shell out big bucks for a FA safety.
But, to answer your question, if it boils down to either/or, I'd say "Shields."
Reasoning: (i) I think you need three really good corners, and I'd rather have a CB playing nickel than a S. (ii) if the front seven gets their act together (big if, I know), they can cover up more S problems than corner problems. (iii) if the front seven doesn't get their act together, it doesn't matter.
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
Romans 12:2 (NKJV)