I've always worn the same helmet for mountain biking and road riding. If they come with visors, I take them off cause I don't like visors. It's a personal choice

I have had Giro Pneumos in the last couple of years - very nice

and I have a Specialized Decibel - very nice aswell

They both have lots of vents and have a good way to tighten the helmet at the back so you can adjust it each and every time you wear it easily. This is very important