That's a tough one
I want Shields. In fact, he's the guy I want the most. But he's a straight up rental for only 3 months. I hate to give up anything but a couple guys among positional player prospects/depth for him. But a guy like Sanchez is probably exactly what they want. Will they settle for less? Probably not. All depends on what others are offering I suppose. And if they can't get a prospect they really like, they'll just qualify him and collect the draft pick.
Presuming he does become a free agent this off season, and especially if he's traded mid season and won't be subject to a QO, I want AA all over him. Would he take more than Anibal Sanchez money (5/80) to sign?? The two guys seem remarkably similar quality wise. Shields is 2 years older but GM's probably value Shields' post-season reputation more, rightly or wrongly.