I have a few hundred hours in the Chippy and every one flown while wearing a Mk IV helmet. I think that I did most things that you would normally do in them, including bagiilions of aerobatics and found there to be no restrictions to movement etc. I am 6' and there was plenty of canopy clearance. Most of my flying was for the RAF Air Experience Flights but I also did plenty of glider tugging using HM's DH aircraft.
I would have no problem recommending the MK IV as a sensible safety precaution but, if I did have the choice then I would chose the Alpha helmet over the MK IV. I say this for two reasons: the most significant one is weight and the fact that the Alpha is lighter. The MK IV is not a problem when you're tootling around looking at the sunbathers but, of course, it weighs four times as much when you're pulling up into a loop and for those who aren't used to having all that extra weight on their heads it might just put them off wearing a helmet. The same thing will happen with the Alpha, of course, but it won't exert quite the same weight. The second thing I prefer about the Alpha is the dual visor system. Now, depending on which mark of MK IV you get, you may well have independent visors and that's a good thing, but the earlier versions required the clear one being locked in place before you could lower the dark one. That's a great safety feature for aircraft equipped with ejector seats and miniature detonation cord but less of a requirement in a Chippy. Of course, it would always help in a birdstrike situation but that would probably be on the rear of the canopy and less of a problem than a faster mover
The American forces helmets (HGU I think) are much lighter again but they were form-fitted to the wearer and had fewer adjustments to ensure it fitted properly and comfortably. They were comfortable by virtue of the fact that they were form-fitted and that wouldn't be the case if you were buying second hand. The MK IV and Alpha, on the other hand, have plenty of bits of string and straps to ensure a decent fit and make sure they stay put when you want to use one of Martin Baker's disembarking systems.
A very close friend of mine is alive today because he was wearing a helmet in a Tiger Moth when it flipped upside-down in a field, following a forced landing after an engine failure. The passenger was not so fortunate, despite having the protection of the wing struts etc closer to them. I agree there's a reverse snobbery at many of the fields we're likely to visit and it is ridiculous to have some sniggering idiot make comments on ones 'coolness'. But let them have their silly opinion. You are going to be safer no matter which helmet you pick, over flying bare-headed and the Chipmunk does allow you the space to be comfortable in whatever you choose.