It's not that one motherboard is better than the other, rather that one is designed in a Micro-ATX format (the extreme-M) rather than the full blown ATX layout (the one without the M).
I purchased the non-M edition because I have a full tower case and am not concerned about space. It also has a few more PCI slots for add on cards later if you use them.
As far as better or worse, they hardly have any differences at all. Gold caps are the same, chipset is the same, software is the same.
If, however, you're interested in TRI-SLI or Crossfire, you'll need the extra space of the non-M edition.
Most important: make sure your case can fit the larger motherboard. If it says "compatible with ATX form factor" then you can buy either.
If you're money conscious, just get the Extreme-M and don't worry about it. You save ten bucks, probably won't ever use 3 gpu's if you're money stingy.
I got mine because I'm putting 3 GTX 670's in SLI.
Have fun building!