Well I do like to look at the Sharp rating when I'm looking for a lid, but any of the big players I think you're pretty safe with. People tend to say that the most important thing is that its a good fit. Thats why I always buy from a shop, as opposed to ebay/interweb. Once I tried one on in a shop, then bought the same one same size of ebay. Sneakt huh ? When it came I swear it did not fit as well as the one in the shop ! So I sent it back.
When I try one on in a shop now after I've picked and the fella gets me a boxed one out the back I try that on too, just incase its any different from the one on display !
Back to sharp ratings. I once read an article about why Arai score so **** on it. one of the impact tests is done on the side the lid, about where your ear is. Arai's tended to be weak there, but Arai say that part of the lid is never hit hard because your shoulder takes the brunt of the impact when you land on your side. They said that their years of supplying racers gave them enough data to prove it, so thats a place where they can save a bit of weight. And who am I to argue ???
go for best fit . . . end of