The boards are virtually the same aside from the # of PCI slots. If you just plan on using one GPU then go for the mATX. However the ATX is a bit more future proof.
Achieving a mild OC (~4.0GHz) is quite easy with the ATX board. I am running a 3570K @4.0GHz very easily with the ATX mobo. I don't see any reason why the mATX would have issues as long as you have proper cooling.
I don't know much about phasing so take my advice for what it is worth. I am relatively new to the PC world.
And good link you provide, thanks.
But could you elaborate a little on the future proof part - I mean, assuming that OC works on the mATX as well is it does on your ATX, and the mATX still being able to 2x Crossfire, what made you go full ATX?
(I most likely won't go any further than 2 GPU's, the "money stingy" part from your link is kind of spot on ;-)
On Newegg the ATX is actually cheaper than the mATX.
I did my first build with a mATX mobo which worked fine. However after upgrading to an ATX I just never even thought about going back to a mATX. The larger mobo often allows for more fan connections (rather than resorting to molex adapters), better spacing and thus better airflow.