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Arai make 2 shell shapes, an "oval" and a "round" a good shop will get you the right fit by selecting the correct shape for your head then fitting the foam. Arai dealers are supposed to stock both shapes in most sizes and several foam size options for the head liner, and cheeks. Shoei has a round shape shell and some pad options. The composite HJC's seem to be acceptable for protection. I would never buy a polycarbonate regardless of brand or fit. I also would never buy a helmet online. You really need to go to a qualified dealer and have the helmet fitted to your head. Since one of my suppliers sells the Arai brand, I had to complete fitment training before we could purchase the Arai brand of helmets. I would find a dealer local to you that has completed the training, (just because they sell them doesn't mean the sales person you get has been through the training) and go in for a fitment. I know that Alan at Moto Market has a lot of experience with fitment. The process should take 30-45 minutes. If they don't have a suitable fit you will at least understand how a helmet should fit and can make a better choice among other brands. I would always go for the best brand with a good fit rather than a discount brand with a perfect fit.
Lastly the service life for a helmet is 2-5 years. Five years if you rarely use it and keep it clean, in a bag, and in a closet. Two years if you wear it a lot as in 2-5 days a week for two seasons. And of course have it inspected after every crash. You can crack the shell and not knick the paint, send it back to the manufacturer and they will inspect it for you.