Go to your local dealer/store and try them on. Get the one that fits your head.
Not all helmets fit the same.
Also, I try to buy from local dealer as much as possible. They give me a discount. If you ask nicely, a local store will probably match what an online store is selling for to get your business, if not undercut them - Repeat business is a big incentive for them to look after you.
Support your local store and they will support you.