I have raced with both, and I prefer the full-face helmet over the MX style for several reasons. One being there is more coverage from roost/rocks/debris and weather with a full face helmet. I wear mine at the dunes because I hate getting sand all over my face, too!
Less windnoise for your microphone in a full-face helmet.
As for fogging up....that depends. I ran without my pumper all weekend at the dunes in my HJC, and had zero fogging issues. It has good venting already (nice big top-ports and vents in the front), so as long as I was moving, I got plenty of air-flow.
You probably have a better chance of fogging up with a skirt and no pumper than without the skirt. The skirt will trap alot of your heat in there......I have raced alot more races without a skirt than with, and I can't really say I prefer a skirt anyway.
Also, I have had other helmets and no pumper, and have had some fogging issues on very cold mornings/nights. So....I think it depends on your helmet and it's venting more than anything.
Full face is also safer......I have heard of two people who have shattered eye sockets/broken noses from a rock being kicked up at them while racing with an MX style helmet. Goggles protect from the little stuff, but a face shield can take a lot more abuse.
Any reason you don't want to run a pumper? I didn't want to run one originally either, but now that I have used one for so long, I don't like being without it.
Hope that helps a little!
