Have you shot any of these lightweight J frames before? I have the shrouded hammer 638, and std 38 specials get old in a hurry.

I got a 640 Pro late last year, and I love it. I now carry it owb and iwb instead of my M&P Shield 40. They weigh about the same.

Depending on how you intend to carry it, the added weigh of the 640 may be negligible as it is for me. The 640 gives me a more durable finish, much more fun at the range, and the option to shoot 357.