I have a few M19s and they are great guns that I have bought when the prices were below $225 for a decent Combat Magnum. They are good guns but not really made to shoot a lot of powerful .357 Magnum loads, particularly not with light jacketed or semi jacketed bullets.
The S&W 686 is a much more solid gun while still giving the impression of a refined and well balanced gun.
If you have your guns to shoot them and need to choose in between the M19 and the 686, my advice would be to keep the 686. I am glad that I do not have to make that decision!