Hi 500 man
Remember to think what you want the helmet to do. I consider my biggest risk, being formation flying and having a collision. Then bailing out and hitting my head on the tail. (thanks Rob Davis for showing the Strong model 304 is a good purchase).
The following is from the gentex website.
CAUTION_ HGU-33, HGU-55, HGU-55/E and HGU-68 helmets have been designed for use in fixed-wing aircraft. They offer limited impact protection. Users with other applications should consider helmets with greater impact protection designed specifically for their application.
They are designed to protect a pilot in the event of using the ejection seat.
I have an Alpha Eagle, Gentex and DC bonedome. If it fits in the ac with me I will use the Eagle due to its significantly higher impact protection. The Gentex is great, but when it comes to protecting your bonce its in a different league. The Gentex is cheaper for a reason.
HSL have gone down hill since they were taken over(own personal opinion), but I believe Alpha have taken over direct marketing.
When flying with a pax I think its nice to give them a bonedome and chute too. However now the price really starts to rock up.
Good luck, but do think about what you are getting for the money. I have been through this in depth and I would never buy a second hand helmet. If they have not been cared for, the impact protection can be greatly limited. A simple sand and pait does not repair the integrity. Sorry to complicate your decision.