From what I have seen, Swisher has a solid throwing arm. I have also read reports that if Swisher made the switch to 1B, he would become one of the best defensive 1B's in the game.
As for Herrera, his value is mainly tied up in his excellent defense. Good range and a cannon for an arm. He would start in CF over Milledge I would say. But this trade isn't happening, so whatever.