I haven't really compared in a long time, but it used to be that mtbike helmets came down lower around the back of your head. I think the theory is that you are much more likely to go over the bars and land on your back, or just onto your side where there are rocks, etc., so you need more protection.
I actually use a mtb helmet on the road but may be weaning myself from them. I love the visor because it helps protect my eyes and contacts from the sun and wind. But on long steep road downhills, I have to hold my head higher than normal to see way out in front (like AG said). I end up pulling off the visor and sticking it down the back of my shorts or someplace weird and forgotten (like Snap's car) so that I can safely make it down the hill. I also learned that I can't see from under the visor when I'm using aero bars, besides it probably slows you down in a TT (I'll use ANY excuse).
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.