You also need to take into consideration the fan favoriteness (nice word there, right?) of youk. Mueller doesnt have a "mooooool" chant going or anything, but youk has his "yoooooouk." The fans at Fenway love Youkilis, more so than they do Mueller, and although you really can't make a decision based on that alone, it may affect how they each play.
Youkilis has said that he feeds off of the energy at the park, and how that youk chant really gets him excited. Mueller wouldn't get that type of reaction. So, why not keep a super-hyped Youkilis in there? Plus he's a prospect, something that the sox are in desperate need of. My signs point to Youkilis right now.