I wear a snowboard helmet in winter, for the earmuffs that are in it. You can remove em in the summe and make it a bit cooler, but here in the desert I barely want to wear a regular vented bike helmet in summer. Just depends on the climate. The boarding helmet is obviously made of more stiff material than the bike helmet, but I couldn't say if it protects more or not. The snowboarder is expected to ride into a tree though, and speeds of 50 mph are easy to reach. The bike helmets have come a long way since they appeared in the 80's, and are designed to blow to pieces in a bad impact which actually lowers the g loading of the skull like crush zones in cars. A motorcycle helmet does the same thing, but is designed for 50 mph rather than 25. So maybe the motorcycle helmet for 50 mph, bike helmet for 25mph, and snowboard or DH helmet for 30-40 mph?
Bottom line, the worst bike helmet beats the crap outa no helmet. I should have photographed the one my head crushed into a curb. Saved my ass for sure. Huge dent in the side of the helmet.