I've found that I can't ride DH with anything BUT snow goggles...
I bought some O-Frames a few years ago and they fogged up halfway down every run at Whistler, whether it was sunny or rainy/overcast.
I tried new lenses, Cat-Crap, other anti-fog, taking them off every second that I wasn't moving, and it still didn't work.
The lenses in MX goggles tend to be pretty flimsy/crappy quality too.
SO I bought some Oakley Crowbar Snow goggles for boarding last winter, and with the dual-pane, vented lens, I still would fog up occasionally if I were hiking in powder or something, but as long as I took them off between runs, they'd be fine.
I now use them for DH riding too, and even on hot days they've been fine and haven't fogged.
SO in short, the snow goggles keep you no warmer/cooler than MX goggles, but in my experience, they fog way less and have nicer lenses (pre-shaped spherical ones vs. a $2 sheet of lexan)