Picking a motorcycle helmet does not have to be complicated, but it is not just a fashion statement either.  Consider that the helmet's function is to protect and then select the degree of coverage you prefer - full face, modified which allows the front section to swing up, open face requiring goggles or glasses or cruiser which offers little coverage for the lower portion of the head sides or back.

The helmet needs to fit comfortably without excessive movement or restriction, so that it can be worn easily for those longer tours.  Also, if you plan to have communications installed, be sure the helmet is designed to accept earphones and microphone.  As to ventilation, having opening ports in the felmet allows air to circulate to provide cooling when needed.   With full-face and modular style motorcycle helmets, you'll likely be happy to have some means of keeping the eye-shield from fogging up in damp or cold weather, by means of fitting an inner-shield.   Finally, the strap ends which attach with an adjustable buckle are far superior to a simple D-ring type in terms of easy attachment and reliability.