A Phenom II X4 965 will perform close to an i5 750, but the price will also be roughly the same overall (when considering CPU/Motherboard combinations).

Mainly the # of PCI slots difference

If you need the extra PCI slots (i.e. you have wireless adapter, aftermarket sound card, tv tuner, etc etc) then you should go with an ATX board that has all the slot you need, otherwise, i would just go with mAtx so you have the option of using both mAtx and Atx case.

Btw, I "THINK" most enthusiasts mb that are good for ocing are ATX.