Oooooooo Kaaaayyyyy.....
I agree that my 15-2 is an especially sweet shooter and I certainly like it and shoot it a lot.
buuuuut, A 4" 686 is worth $500 all day long. And the 15's go for about three-and-a-quarter to, maybe, four hundred beans.
At the LGS, shows, etc I rarely see either with the 15's being rarer (I've seen two in the last year around here - including the one I bought) than the 686's (less than half a dozen in the last year including the local Cabela's 'gun library').
That said, knowing what I do now, I agree with all the other posters = keep the 15.
But if the OP really WANTS that 686, then it, technically, is a square deal for him.
Oh, and welcome to the forum!