I have both, the 686 is a tad heavier, which has plusses and minuses. I kind of prefer the trigger and action on the older K frames but it's a very slight difference, not enough to make me lean one way or another.
I also prefer stainless guns in general, but like a lot of people here, I have the sickness so have 19's and 66's, 686's and soon to have a 586... but that's a different story altogether.
As far as rareness, believe it or not 686's are rarer here used than 19's and 66's. They mad a huge number of 19's, they really arent THAT rare. Findin one mint with a box is another story, but for a shooter, you can readily find them.