Its an appearance issue to me....and I just don't want any S&W revolvers with locks...but I also prefer the older versions where the barrels were pinned and the cyclinders were counterbored - like in the model 27's the dash 2 engineering series. The dash number is the engineering revision on the model...
The 686 Plus --- is a 7 shot version of the model 686 ...started with the 686-4 series in 1996 according to my S&W book ....in 1997 it became a dash 5 where they changed the frame design, changed internal lockwork, floating firing pin with flat face hammer, MIM trigger ....in 2002 it went to a 686-6 and it has the Internal lock system that most of us prefer not to have...but MIM parts are not desireable to collectors/shooters either often.
You'll find the 686 7 shot plus ...was made in 2 1/2", 3", 4" and 6" barrels.../ with the 686 in a 4" probably being the most popular length ( easier to draw from a holster than a 6")...and longer than the 2 1/2" or 3" so you have a longer sight plane...making them a little easier to shoot beyond 21 feet or so...( in my opinion ).
note: all from 3rd edition of S&W catalog .....the only 686's I own are a 4" and a 6" ...both 686-1's.../ and they bear the M mark as modified at factory ...both nice guns / 686's are L frames...between the K frame ( model 66's or 19's ) ...and the N frames ( model 27's, 28's, and 627's ) ....all .357 Mag calibers. When you see the 6 in the model, you know its a stainless gun....66 is stainless K frame, model 19 is K frame either blued or nickel....( the .357 mag makes up most of my S&W collection of model 19's, 27's, 28's, 66's and 686's --- but my favorite of the bunch is the model 27's - the N frames) but they're more money than the 686's../ the 686's are a good value right now if you can find a good clean one.
Good luck with the continuing process of finding the gun you like !