The cylinder of a Model 627 will adequately hold up under .357 Magnum use far longer than you will.
Every gun manufactured by reputable manufacturers is proof tested to withstand pressures greater than you are likely to subject your gun to, using "proof test" ammunition. High pressures are usually not the source of gun breakdowns, in my experience. Most of the problems I have encountered are the result of battering from recoil, and that from heavy bullets.
As to a preference between the 627 and the 686, I have come to the conclusion that the L-Framed Smith & wesson is the best DA revolver in .357 Magnum caliber. I like its slightly smaller frame and longer cylinder, and absolutley love the full lug barrel. It is not just an enlarged K-Frame, but a beefed up heavy duty intermediate sized frame.