Actually you can make the argument that Salty is the better defensive catcher, but the Sox are losing a ton of offense this offseason with the losses of Ellsbury, Napoli, Salty, and Drew(7th in the major leagues for ss with a 109 wRC+). A full year of XB, a bounce back year for WMB, and a healthy Pedroia will help but I expect regression from Nava, Victorino, and Ortiz(eventually he has to slow down). Santana had a better wRC+ than Napoli and he gives the Sox a bridge year at Catcher and is potentially your 1B of the future. While I'm a huge supporter of Doubront, I don't think the FO is a big fan of his.
edit: just saw the Guardians have Santana working at 3B in Dominican winter ball.