Perfect Pass is just a more well known name brand. Perfect Pass "Stargazer" is their GPS system, which is pretty much mainstream at this point and I believe they're phazing out the paddlewheel systems. Zero Off is only GPS I believe, I don't even think they make (or made?) a paddlewheel system. Hydrophase also makes "Ridesteady" which is a paddlewheel only system (and I think geared towards wakeboarding marketing), and it comes in at a value price of around $800 I believe. Any of them would be fine.
I have PP Wakeboard Pro as I stated, and I've used Stargazer plenty of times. Excellent systems no matter what you get. There's a website, ski-it-again.com, that you can sometimes find used systems on. Otherwise, pony up the $800-$1500, and no matter what you won't regret it.