I have 2 XDm 9mm - the full size and the compact - and I love them both. I have not had any reliability issues with either one of them and I've probably got about 1500 rounds through each at this point. I also find them to be very accurate.
I was debating between the M&P, Sig, Glock, and XDm when I purchased my first one, and the XDm just felt better in my hand. Personally, I don't think you could go wrong with either the M&P or the XDm - it just depends on what feels best to you.