I have 2 1/2, 3, and 4 inch M586/686/696 Smith and Wessons.
If I were wanting to carry "light" I'd go 3 inch. If I was wanting to carry "outside-in-holster" and maybe horseback, in a vehicle, or even walking I'd go 3 or 4 inch, which ever I could find/afford first.
The difference in barrel length may not affect velocity enough to warrant an extra inch.
I do say this. A 3 inch barreled, round-butt L-Frame (mine is a 696) is an excellent handling revolver in my opinion. It is truly a dream to carry and handle.
I would even go so far to say that buying a 4 inch M686, and either buying a 3 inch barrel or cutting down the 4 inch barrel and roundbutting it would be a little expensive but worth it.
As well I have handled a 4 inch M586 that was round-butted. That is also an excellent handling firearm. And it would be very easy to do with a stainless M686.
Good luck on your endeavor.