Ok, so I got both the Hans and Simpson hybrid to try. I had no issues getting in or our of the car with either device + helmet fully attached - a bit of judicious neck bending on the way in, but not an issue really. I definitely agree getting these things on/off while in the car is really difficult though. In the end I've gone with the Hans device (20 degree) because side to side head movement is a litttle better and the device itself is easier for me to get on and off - I just put the Hans and helmet on and off as one complete assembly while out of the car before sliding/contorting my way in. This is with a closed faced helmet.
As usual, it's one of these things that I hated because of the restriction in movement when testing statically in the car, but when out on track today with it for the first time, it was quickly forgotten about. It's a royal pain while driving around the paddock though because you can't look over you shoulder for traffic, etc.