I can help you with that. I sold my Glock 21 to get my M&P9c. I have shot the M&P45, and the grip on it was MUCH better than the Glock 21, and I LOVED that G21.
I'm a Glock guy, but between those 2, I'd choose the M&P45.
What's the issue with the MRP serial numbered M&P's? I've got a line on a few and I need to know if there is going to be a problem with them or not. Some have a MRL prefix others have DSN.