Knock down power is a made up term not supported by physics.
38+p is about the minimum I would suggest anyone carry and is a fine option. Not that you mentioned it but I would stay away from laser sights I think should the worst ever happen they look bad to the jury.
I don't care for the LCR I appreciate a bit more heft but they seem like a quality gun and many people like them.
Smith J frame o the two would be my choice. I would go with the 357 over the 38+p simply because you can use both rounds in the same gun. I like a heavier bullet and a heavier gun though.
I would find a place or people who has them and will let you shoot them and see which you like best. Between the two choices you outlined I honestly don't think you can make a bad decision.