Back when I was a teenager, my father and I shot pin and plate matches every week. We used a 6" 686 highly customized by Karl Sokol at
Karl Sokol Chestnut Mountian Sports Gunsmithing Services . I still have it today and shoot it occasionally.
On the other hand, I have 20+ K-frames that I shoot all the time. I've competed and taken revolver classes, always using my K-frames. I have large hands and still find the K-frame fit my hand better and also seem to balance a lot better.
Now, about the .357mag loads beating up the K-frames too much. Yes, if you are going to shoot a lot of them. Back when I turned 21, Karl built me a 3" M65 for a carry gun. I shot that revolver, and pretty much only that revolver, for several years. I always shot my 158gr cast bullet reloads. Back then, when I was much younger and dumber, I didn't believe in shooting anything but max loads. After years and years of doing that, and tens of thousands of absolute max reloads, it was really starting to loosen up, and all that Bullseye powder was causing quite a bit of flame cutting on the face of the cylinder and topstrap.
I'm not saying that an L-frame would not have the flame cutting, but after 4 or 5x more rounds through it than the M65, the 686 is still as tight as the day Karl was done building it into the competition gun it still is today.
If you are going to shoot primarily .38's and occasionally shoot a box of .357's, I'd recommend going with the K-frame. If you want to shoot mostly .357's, you really don't have much of a choice. If you are a high volume shooter, you will shoot the K-frame loose.
Whatever you choose make sure you get a pre-lock example. I've heard it said that only lightweight S&W revolvers have an issue with the lock. NOT SO! I had an all SS S&W revolver lock up while firing target loads. I will never trust a S&W with the internal lock to protect myself. Either buy used or buy a new Ruger. New S&W's are a liability and lawsuit waiting to happen. One day, someone will attempt to fire to save their own life and need more than one round. Unfortunately, they will only be able to get one off and the responding LEO's will find them with a S&W revolver in their cold, dead hands with the internal lock engaged. Hell, even the *** bad guy who kills them won't take the piece of **** since it locks up while firing!
That's not some sick fantasy, it's going to happen. And with the internal lock such a well know problem, when it does happen, it's going to be the end of S&W as we know it.