I always believe a very simple saying "how much is your head worth" but more to the point it has to fit properly not loose or to tight so it does not distract you from the job at hand ie staying alive, it also has to give good vision out to the sides, some modular's don't so the important thing is to check by actually trying them on and checking, some helmets suit different shaped heads as well so don't think because you take a XL in one brand that you take an XL in another brand try try try till you find the one that suits, remember your choice may save your life one day