While he has shown improvement, Duran is not a good MLB centerfielder. By all accounts, Rafaela already is -- and as a prospect who has hit at elite levels on his way to the Show.
Duran's best position for Boston may be LF, but so is Yoshida's, and even if the latter gets to DH a lot, he'll still be roaming pastures somewhere. If I had to choose between trading one or the other, I'd like Brez to deal Duran at what may be peak value to optimize his return. I don't think Masa has reached his ceiling yet as a big league batter, and is worth keeping for now (his contract would also bring back less trade value).
As for Refsnyder, he is the perfect bench-warmer and a happy role-player, and no poster should waste any more typing time complaining about that.