These guys have summed it up pretty well. Basically.
Street: designed for faceshield, very aero for speed, no bill, relatively poor venting, close fitting around mouth
Dirt: designed for goggles, less aero, bill, better venting (dirt riding is more aerobic activity), looser fit around mouth
Adv: cross between two with but basically a dirt bike helmet with a face shield added
An adv helmet is a nice way to go for road riding but the bill and frontal projection does make more subject to aero issues. If you tilt your head back or to the side, you can sometimes really fell the pull at high speed, so its hard on your neck muscles. But the bill is nice for the sun and the extra venting is welcome when it is hot. I also hate how hard it is to clean the shield on my older XD (bill gets in the way) but perhaps they've solved this problem on the new ones.
- Mark