IMO, it's really hard to project how either the Red Sox or Yankees will do this season. There are just too many "ifs" with both teams.
I think both teams are in a similar spot with their starting rotations. Both could potentially be very good (Yankees having more potential than the Sox), but that's provided that a lot of things go right. While I don't think either team needs to add a top of the rotation guy, I think that both teams should.
In terms of bullpens, Yankees look very good.
In terms of offense, both teams struggled with scoring runs last season, and struggled with getting that key hit when they needed it. I think (hope) the Sox have addressed that problem. I don't think the Yankees have. Then again, the potential is there, I suppose, but with a lot of "ifs".
Two additional factors for the Yankees: 1. What effect will Arod have on the clubhouse? 2. Everyone knows how lucky the Yankees are. ;-)