I'd hate to see MartyB go, but if you can get a high quality pick for him, I guess there is no harm in considering it. His departure could open up a roster spot for a more versatile and potentially dynamic player (e.g., Dorin Dickerson). I still think MartyB has a big upside, though, and would prefer that he realize his potential in Dallas.
I think that Choice and a rookie short yardage type back could take over the MB3 role quite nicely. I'd be o.k. with trading MB3 away so long as the Cowboys get at least enough for Barber to draft their new power back.