Compare weight and ask about wind noise, pay attention to visibility from inside. How well do you see out in your riding position? Look very very carefully at crash ratings. Some are rated in several countries which is a big plus if you ask me.
There are "comfort helmets" made for low speeds with soft thick inner lining and there are more race oriented helmets that are much harder in the lining to protect at highspeed crashes. There are some big considerations knowing how you ride, where etc. High quality highest crash rating are going to fit like a glass slipper. As in, it fits or it does fit. You should try a bunch of helmets. Wear them for 5-6mins each minimum to see if you fit. There are some pretty long threads showing everyone's favorite helmets. You should look them up - there's a lot of info on some of those threads. Cool images as well.
