I've used the Bodyguard series of revolvers for a quarter of a century. My experience is that when carried in a pocket holster in substantial trousers such as jeans supported with a sturdy belt, the difference between the alloy frame and steel frame weapons is unnoticible--and I'm the quintessential 97# weakling. I'd be more concerned with getting a weapon with a wide front sight than a alloy frame.
I've never understood why one would want to carry a pocket weapon in a belt holster, when something better could be carried with no extra effort--but to each their own.
Something I've never seen discussed about the Bodyguard frame weapons is that the humped frame supports the web between the thumb and index finger. Furthermore, the hammer shroud prevents hammer bite. I have small hands, and this is a problem for me with exposed hammer J frame revolvers.
My chief complaint with the Model 638 is aesthetic--its a perfectably servicible weapon, but it appears to be coated with factory-applied congealed snot.