A little more information may help the .357M decision-making process easier. First, the Rugers are all cast SS, while S&Ws are still hammer-forged and heat-treated. Second, in the actual comparison of the 'equivalent' models, the 4"/6" SS GP100s (KGP141/KGP161) and 686s (164222/164224), the weights are 41/46 oz and 40/44 oz respectively - not the huge difference inferred by many. Plus, it just takes more cast steel to equal the strength of hammer-forged and heat treated steel. Sadly, much of the GP100's added weight is in that massive barrel shroud. Of course, want a 7-shooter? S&W!
S&W's don't shoot themselves 'out of time' any faster than any other maker's .357M. The true Achille's heel of the K-frame .357M's, including the SS 65 & 66, is the minute and edible forcing cone - the hyper-hot 110 & 125 gr loads do etch away at that dimunitive edge. Still, I've never seen one bad enough to warrant a barrel replacement. The same with the supposed topstrap flame-etching. Neither are a problem with heavier loads. The L-frame was developed for 'insurance', it's front frame being thick enough for .025+" more barrel diameter, and the taller frame opening permits the 7-shot .357M cylinder - and a 5-shot .44 Special, too.
As long as the 10, 64, 67, and 617s exist, there will be a K-frame. The 65 & 66 were 'replaced' by the 619 & 620 - which are actually L-frame bodies with short-lugged and modern two-piece barrels. The K&L frames take the same grips. I like the short lug look, feel, and pointability, muzzle jump being less important to me than the 'pointability'. Sadly, Ruger did away with their short lugged GP100s some time back.
If you are a normal SAAMI-spec .357M commercial ammo shooter - or reloader who doesn't 'test the envelope', the S&W will last a lifetime - and with a smoother lockwork. If you find any fault with it, you have a pre-paid shipping label as close as an 800 call - for 'life'. Ruger lets you pay for the call and shipping - and doesn't specify a term for it's warranty. Still, there is enough to mull over - and I predict you will like whichever one you buy.
Stainz