I recently picked up an Asus M4A785-M, which is their cheapest 785G board, with an Athlon II X3 435. Without really pushing it, changing voltages, or using anything besides the stock cooler, I got the processor up to 3.4GHz in about 5 minutes *shrug*. I don't know if I accept the argument that ATX boards overclock "better" than mATX boards.
I don't think there really is a huge difference, it's mainly if you'd rather have the extra expansion slots provided by the ATX board or not. All motherboards are made using the same types of components, and these days reputable manufacturers seem to make quality mATX boards as a viable alternative to ATX boards, not just as a cheaper (price-wise, not quality-wise) alternative. That being said, any really high end motherboard is almost certainly going to be ATX, because there isn't much demand for a super high-end mATX board, since most people who are into that kind of thing want a full-sized board (extra expansion slots, perceived better overclocking etc).