Pprune search for 'Gentex' should lead you to all the relevant threads. I use an HGU 55 - works well for me in various aerobatic kit, including the Extra. The Alpha provides slightly better protection, but is more bulky. I know of some conversions of motorsport helmets with kevlar shells with aviation comms kit, which offer substantial savings over dealer prices for Gentex or Alpha, but if you can source a new HGU 55 shell then fitting the liners and comms yourself can be a lot cheaper as well.