In terms of impact feel of the material, I don't think it would make much difference to the recipient between a hard polymer and steel. Neither is going give when cracked against ones skull. It's more a question of leverage. So, is a fully loaded magazine of whichever semi-automatic pistol heavier or lighter than the slide, taking into account the distance from where you're gripping the bludgeon?
I'm guessing that in the case of a 1911, you'll do the most damage holding the slide and whacking away with the bottom of the grip with a fully loaded magazine.