Go to a shop and try one. Helmet sizes differ with manufacturers. Also everyone's head is different. As you will be wearing it in all types of conditions and time lengths choose comfort over style.
I've one of these, I find it very comfortable and the strap system works well.
Well worth the money, but worth trying on before buying to make sure it fits your head
I went to go a try a Hex on today, it was snugg, but it felt really hot and I only worn it for a couple of minutes and the straps were really uncomfortable when it went around my cheeks.