I just don't "get" how 2000 rounds per year is considered a "high round count", I have months where I'll send 2000 rounds downrange and some consider me a lightweight.

In regards to the steel used in the 686 series it's either type 416 or type 436 Stainless Steel, both of which have properties that basically match 4140 "chrome moly" steel. Meaning it's an extremely strong and durable steel that is also resistant to corrosion.

In theory the CYLINDER for the 686 Plus is slightly stronger because the stop notches are between the chambers instead of at the thinner spot created by the chambers. However IMO that "weakness" would require a hand load operating about 50 to 75% over the Maximum pressure for the 357 Magnum to be considered a potential problem. Basically use sane hand loads or standard commercial 357 Magnums and it doesn't matter which cylinder you choose. Personally I like the extra capacity of the 7 shooter but every time I go to the range with mine I find myself dumping a loaded round into my hand for at least the first 3 or 4 cylinders because I seem to have counting to 6 tattooed into my brain.

As for the FRAME, ALL L frames use the same, identical, frame forging. So there is no difference at all between the 686 or 686 Plus in the frame strength. I also expect that there is no difference between the blued 586 and the stainless 686 but must state that I am assuming the 586 frame uses 4140 steel. If the 586 uses 1040 or 1020 the 686 is actually slightly stronger.

Finally, by many reports the 686 can last between 60,000 and 100,000 rounds before a loss in accuracy due to a worn out barrel is seen. So, at only 2000 rounds per year you have between 30 and 50 years of "life" in a new 686. The only defect I see in the 686 is that full length underlug, IMO they produce a barrel heavy balance and are quite UGLY. To bad the 686 Mountain Gun wasn't the standard model and the full underlug imitation Python was the special edition only seen once.