Interesting. You are the first person who has advised against this. I hope more people offer their opinions also.
While I really like Ruger single action revolvers and probably shoot them more than any other firearm, I would have to agree that it's not the best substitute for practice on a double action snubbie.
I have an older Colt Agent that I slowly got better at as long as I was firing it signle action, ie pulling the hammer back and then pulling the trigger. As soon as I tried switching to double action, the little revolver moved all over the place.
When upgrading to a new snubbie, I went for the Ruger LCR, which is double action only. This way I not only stay consistent by always shooting double action, but the double action on it is much easier than the Colt ever was. My decision was based on the assumption that I would be firing double action only if I were in trouble.
I realize that a 357 is spicier than a .38spl +P, but I'm sure a lot of that is lost when it comes to such a short barrel. I can shoot .44 magnums out of my big Ruger without problems, and a .44 magnum out of a 7 inch barrel is an outstanding defensive round. However, I am not as skilled as others here as concealing one. That leaves me with pocket carrying a 357 or 38spl snubbie.
I found shooting 38spl out of the LCR is WAY more manageable than firing .357 out of an airweight, so for me an LCR or other double action only snubbie in .38spl makes more sense.
If you can handle a lightweight 357 as well as a 38, then go for it by all means, but for me, the additional recoil, noise, and flash just made it that much harder to control followup shots.