I would highly recommend the Extreme Engineering patented auto belay. (http://www.extremeengineering.com/ for US and www.veqtor.co.uk for UK and Europe).
They are the original creators of the auto belay and have a 100% safety record which they've held since it was designed 15 years ago.
They are used by Disney, Six Flags, Cirque du Soleil and the US army.
Hydraulic systems are definitely best, as the other competitors (like red points) are fall arresters which means as soon as you've had one fall on it you're supposed to send it in to be serviced as it has served its purpose.
Also I've heard that the North Safety belays have problems retracting and customers often get covered in break dust, and the wires get tangled.
But with the Extreme auto belay it's impossible for any of that to go wrong.