I haven't followed Duran, but looking at the numbers, I see a good year last year and a really good year the year before. He's already 29. It's a little hard to know if these last two years are harbingers of greatness ahead or, given some of his lesser production last year, a blip.
Looking at Peralta, he's gradually gotten better each year, was really good this year, 29 years old, and is a more rare commodity than Duran -- solid, healthy, proven SP (one of the NL's best), let alone on a really good contract.
I don't get how a one-for-one's out of the question. A 3- or 4-for-Duran seems absurd -- an easy & hard no -- if Peralta goes the other way.
I can definitely see how Duran would be an upgrade in the OF. At the same time, I'm not at all eager to get rid of Freddy, even if the odds of keeping him long-term aren't great, because the rotation should definitely be much better if he's in MIL.