Headroom and cost are factors. If you're flying a Pitts, chances are you won't get your noggin and an Alpha under the canopy, certainly when pushing negative or are inverted. So that's one factor. Another is cost. It's been nearly 10 years since I bought my helmet but, if IIRC, back then the Alpha was 50% more expensive than a comparable rig on the HGU-55. I reckoned that the -55 was "good enough" for the money, so went with that. I've been pretty happy with it, but the passive noise attenuation in the Pitts is a bit lacking, so I wear earplugs when I wear it. YYMV, but I'd suggest the HGU-55.