I do like my SLI/Xfire setups tho. You can get some one-generation-old high end GPUs fairly cheap where I live, and while multi-GPU setups are far from perfect, it makes sense to me. Because of this, I'd go with nothing smaller than mATX. I really don't mind having a larger case, and the extra options that are afforded by ATX and E-ATX boards is well worth the extra space.
I understand the whole SFF thing, and there are some really cool little builds out there, with a LOT of horsepower under the hood. The cost of these builds is just out of my reach though. It seems there's a trade-off between performance and form factor, and I'd rather have the extra performance, given that I have the room for a mid or full-tower.
So, long story short, I'll most likely be going with an ATX board for my next build.