If you do want the lower-jaw protection, try one of these (Ruroc helmet):
1.) They're actually designed for snow sports and are much more low-profile, so it will hardly affect your performance.
2.) The lower section is removable, so you can easily turn it into a "normal" looking snowboard helmet.
3.) The goggles do not fog up.
4.) Get it in the right color, and you'd actually look like a bad-ass!
This I would condone, cause it does not inhibit your vision of the front tip of the board. I'm sure lateral peripherals are fine, but its the downward peripheral vision that would be sacrificed with the other "MTB type" designs.
I must say, I didn't expect to come back and find this thread sitting at the top.
For those concerned, I've been rocking a Bern Brentwood Zipmold for the past few months. It's perfect for XC riding and I'm sure it will be great on the slopes this year (with the winter liner). Bonus: the color scheme matches both my Haro and my Forum.