Ha, of course I'd rather have Ellsbury over Span, but I'm also a Sox fan, so I'm conflicted and not sure how both the Sox and Nationals could get better in a deal between them.
I think the Sox are going to want a lot for Ellsbury, not sure if Alex Meyer.. and say Anthony Rendon would've gotten it done.. or if Rizzo would've even considered letting Rendon go. Ellsbury I believe has another arbitration year left, so you'd get two years minimum out of him, but he'd cost a lot more than Span who's solid, yet unspectacular.
Ellsbury also sandwiched two years pretty well lost to injury around an AL-MVP -like season so he'd be a bit of a risk to give up two good prospects for and then two arbitration years at 10 and 11 million per year (no idea what he'll get, just guessing).