S&W still does offer both the 6 and 7 shot L frame 686 series, kind of weird but there must be a call for it.
I have had a 7-shot 686+ 4” version for about 20 years. I shoot stout loads in this gun that I would not shoot in my .357 K frames. It did have a funky crane fit early on so it did go back to S&W several years ago for a repair, but outside of this oddity it has been a great gun.
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Here is something to think about since you mentioned cylinder wall thickness: the thinnest portion of a S&W 6-shot revolver cylinder is the bolt stop notch. It is cut right over the middle of the chambers on these guns, making for a much thinner spot in the chamber wall than it appears.
On the odd numbered cylinders (5, 7 or 9 shot) the notches are cut between the cylinders. So, there may be more meat between the 6-chambers of the 686, but the weakest part of the S&W 6-shot cylinder is still that notch cut in the thinnest part of the cylinder wall.
Still, the 686 is a tough gun.
Rapid fire DA, the 7-shot 686+ trigger pull seems a little bit faster than the 6-shot guns, I guess it is moving the cylinder a bit less with each DA trigger pull. Plus, adding around 16% more ammo carried in the same sized package (7 vs 6) is not a bad thing.
Both are good guns. Personally I like the +, but if you want a touch of six shooter nostalgia the 6-shot 686 will do you right.
Stay safe.