My thought is that either one will serve you well. I don't have n SP, but I have a short barrel Speed Six in 38 Special. I also have a 4" K-frame Smith in 357. I have never shot 357s out of the K-frame. My larger and longer options get about a 50/50 diet of 357 and 38.
"And the government promised me a posse, which I figure will be long on promise and short on posse." - Rooster Cogburn