In the Shay vs. Brad debate, what it comes down to for me is this. I think Brad's had a pretty bad season, at least by the standards he set for himself in the previous couple of seasons. I think this is largely due to the fact that under Lambert, there was never anyone pressing him for his starting spot and that he grew complacent and never seemed to be seeking to improve.
Given, though, has all the weaknesses that have already been pointed out here -- weak at crosses and high balls, allows himself to be bullied during set pieces, etc. In my view, neither of them are ideal (Brad of two seasons ago would be ideal).
So the deciding factor for me is how the rest of the back four look depending on who's in goal. Our back four, in any given combination, always has a tendency to panic and give away cheap goals, especially late ones. But they look just that little bit more calm, more assured, and less panicky in front of Given. Maybe that's due to his experience, maybe he communicates better, I don't know. But they seem to perform better when Given is behind them, and that makes a big difference.