I use wood. It's best on my knives and naturally inhibits bacteria growth, or so I'm told. I've also heard that the plastic boards, once scored, trap bacteria so that you can't really get them clean.
I use wood, but different boards for different types of foods.
CBJ, some comps, a few awards, Certified Artisan Steak Taster.
XL Big Green Egg, Bubba Keg Convection Grill, Weber Kettle, Charbroil Infrared and mad livefire cooking skillz! :)