Bay isn't the problem for the Sox.
But if we're going to compare the two, despite his blunder in the NLDS, Holliday is definitely the better outfielder. Holliday is also a better pure hitter and gets on base at a pretty good clip. But then Bay also gets on base at a good clip. They can both steal bases. They're about the same age; Holliday is a little younger.
If I had to choose between the two, I would say Holliday is the better overall player. However, I'm not sure who would come cheaper.