I got the AirTurn Duo 500.
The primary difference with the cheaper pedal I'd say is the action. It's exactly as Harriet Wong explained it in one of the videos above. I like the new action a lot, although realistically-speaking, it's a subtle improvement. You could certainly survive without it.
The second difference is AirDirect. This lets you pair the pedal with the app directly instead of the OS. So if you don't like the annoying side-effects of pairing an input device with your tablet, this is for you.
However, if you use AirDirect, then you naturally lose a nice feature on a dedicated setup -- you can't wake your sheet music reader just by pressing the pedal.
So I went back to normal pairing with the OS. That ended being a Bluetooth nightmare though it was not the fault of the AirTurn and all is finally good.
Another nice perk is that the AirTurn has a dedicated app that lets you do some advanced customizations.
So the AirTurn works, it's nice and I'll get to enjoy it some more, the more I practice.
The Donner is good too.