ATX v mATX

If you plan to SLI/ Crossfire and case size is not a issue ... then get a ATX.... give you more space to expand..

Is there any other benefit to ATX other than the SLI/Crossfire ability ?

Some M-atx motherboards do have sli capability.
The problem is, that the two cards which are normally double slot cards will be installed next to each other. That causes heat issues for the top card.

If you have the need for more than three expansion cards, then ATX is for you. But today's motherboards already come with integrated lan, sound and sufficient usb and sata ports, that most of us really need no expansion slots other than the pcie-x16 slot.

Some of the enthusiast boards intended for maximum overclockers will tend to be full sized.

If you really want a smaller system, you might even consider a ITX based system. Look at the lian li Q08 case for a great lan party pc.

I hear gaming, I assume CPU OC, and most mATX MOBO's have insufficient power (phases) to the CPU; answer stick with full ATX. Between expansion, confined space aka too much heat, and OC'ing IMO get an ATX with a Mid Tower case.

If your case only allows mATX/uATX then obviously the smaller MOBO.

The only justification for a 'smaller' size form factor is a (portable) 'LAN Part PC', (edit) or HTPC.

I have another justification. My desk space limits the depth of the case to something < 16"

In my current case, a Silverstone TJ-08E, I run cooler than a previous full sized tower, and can oc comfortably to 4.0 which is all I need.

ATX vs mATX is HEIGHT not depth. If the case is too deep then it has zip to do with the MOBO.

True enough.

But the only mid-size case I could find <16" deep that could hold a full length graphics card was the lian li X1000. It was a great case, but a tall monster.
There may be some cases available now that will fit the bill if you can remove part of a hard drive cage.

Regardless, the OP wants a smaller system with no cf/sli.

A Micro ATX motherboard is generally the same as a full size, they just don't have as many expansion slots. As far as performance goes, the Micro is going to be the same as long as it has the same specs on each component. Meaning, if it has a x16 PCI-E Slot, a standard FSB speed, Dual Channel Memory, SATA etc..
if only planning on running dual GPU's on the board and nothing else installed in the PCI slots then odds are you can use the mATX board.
if planning on dual GPU's (SLi) and maybe adding another plug-in card of some type: TV-tuner, wireless PCI-E mini or something
then you need to play attention to the motherboard layout and more so if going mATX.
odds are the ATX can be considered future-proofing.

@geofelt - That still doesn't change my advice. Folks don't know all the ins and outs, that's why they ask questions.

I 'get' the wanting small, but I too 'get' the cost of doing it as well -- same 'HEAT' confined in a smaller >space<; solution get higher CFM + noise fans. Even a single GPU puts off a lot of heat, not to mention the MOBO + CPU even @ stock.

I run across this situation frequently, and both the Pros & Cons need to be addressed.

I can tell folks only what they want to hear <or> I can choose to tell them what they need to know. I choose the later.