Sorry to complicate your decision.
Yeah thanks! I agree with you about second hand helmets. I found an interesting research report on the Oregon Aero upgrades for the Gentex helmet
http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=ADA383420. The upgrades aren't very expensive, and it sounds like they're worth it.
From my perspective I'm looking for hearing protection first and bonce-protection second. Every time I fly the hearing protection will be useful, and maybe I'll never have any need for the helmet, but it could be useful. I don't think it's easy to predict where you may smack your head or how it may be crushed in a light aircraft crash, so I really don't know whether the Gentex would be sufficient or not.
I can't really justify the cost of the Alpha 900, and I'm not convinced that it costs that much more because it's that much better, or if it's down to the fact that it's sold by only one distributor.