What's the difference? If you are not in Time Trial do you need that aero design at the back of the helmet? If you are climbing in the heat won't a mt. bike helmet offer as much comfort and ventilation as a road bike helmet?
I bought a slick looking Uvex helmet with a visor, precisely for multi purposoe riding (Road, MTB, commuting). For use on the road bike I took off the visor and turned out looking like Dimspaces's avatar.
I don't mind the look but what a difference the visor made on the looks of the thing.