Myself I would prefer better defense on the left side if XB is going to be the SS alongside WMB, I am not convinced they will hit so good as a pair that it will offset the defensive weakness they will create on the left side. I would prefer to see WMB continue on the path as a power hitter that he was on and move over to 1st, have XB at 3rd and a competent SS, at least in the sense of what the Sox consider to be a competent SS.
I just do not know what WMB did in the minors to convince people he had the potential to be a solid defensive 3rd baseman. I just don't see it. However I think XB will be able to hold his own at 3rd while being subpar at SS.