I think a 5th starter is a more important need because the Sox do have guys like Munoz, Sanders, Ginter, Wright, Rauch and Diaz that are at least capable. Plus I still have faith in Koch.
If the SOx make the playoffs, at least one of the starters would be moved to the pen anyway. Still the best would be the Sox willing to add payroll to the point that they could get a starter and a reliever for basically nothing.
Or if they could unload Konerko and a prospect for Ponson and Lightenberg.