The main difference is the size. The ''non-M'' is ATX while the ''M'' is a micro ATX. I also did a quick read between specs on the ASRock official website and found the following differences:
The ''non-M'' has a 8 + 4 power phase whilst the ''M'' has a 4 + 2.
Also the ''non-M'' has 3 more rear USB ports, 1 more chassis fan connector,LED on power switch, reset switch,clear CMOS switch and something called Dr. Debug,1 more PCI-e 2.0 x1 and 2 PCI-e lanes.
The ''M'' has 3 way Crossfire(both have Crossfire, Quad crossfire and crossfire X)
I think the non-M is the choice for me.