Ventilation?

Most skateboard/street BMX helmets are solid plastic outers with a few holes in them. Bicycle helmets have more ventilation and also more adjustment fit wise.

All helmets should meet a safety standard (they have to be tested to achieve this) so if the helmet has one it means its safe. A $60 helmet is just as safe as a $200 helmet if they both have safety standards, just one might be lighter, made with carbon bits, looks better etc.

That said Specialized has got a MTB specific helmet that comes down lower on the back of the neck, because you fall differently than road biking- I'm not sure if that is true.