I carry around a 2.5 inch mod 66 in a Tucker Texas Heritage holster. This is a very comfortable carry system. The Tucker is designed to carry at on slightly behind the hip.
The 66 conceals very well in this type of a rig.
If I could have I'd have gotten the 3 incher but it wasn't available when I got this. I'd think that the 686 would be a little difficult to carry comfortably, unless you got a goot two point holster and a good gunbelt, as it's heavier than the the 66 which weighs in at 36 ounces empty.
As was mentioned earlier, there is a BIG difference between the smaller and larger of those you mentioned.
Concealing any gun depends on the gun, on the holster, your belt, your body shape and on your choice of clothing. A good gunbelt is very necessary to hold the weight of a larger gun.
Whatever gun you get, you will probably go thru several holsters before you find one that works right for you. Take a hint from someone who has probably spent more money on holsters than I have on my handguns, don't buy cheap holsters. You'll buy many before you realize that money could have been spent on a high end one in the first place.
Here's my SW CS45 in a Texas Heritage:
This is my main carry rig. I like the way it snuggles to me more than the 66. But the 66 still carries well, just not as good as the CS45 - at least for me.
and my J frame SW332 .32 magnum in a Tucker 'coverup'.
This sometimes goes in a Mka pocket holster.
As you can tell, I'm a fan of Tucker gunleather.