I've consolidated to only two handgun calibers. 9mm and .45 ACP. I want a .45 ACP handgun....Glock 21 Gen 4 or M&P 45 full-size? Why? What are your reasons? I plan on sending my slide to Mark Housel at L&M Precision, to have the RMR package done, so take that into consideration. Pros and Cons of each. I've yet to hear of any Gen 4 Glock 21 issues. I don't like the fact that the M&P requires you to take something and push that little lever down inside the magwell. I've heard of issues with rust on the M&Ps as well, however, whichever gun I buy, I'm having Mark Cerakote the entire slide after milling the cutout for the RMR. Wanna kill these ads? We can help!