I've never had a paddlewheel speed pickup but haven't heard of that many problems with it. That said the GPS puck has no moving parts and the pucks are standard ones that are mass produced by the tens of thousands for various applications outside of boat speed measurement/control. So, reliability is pretty good. I'm sure it has happened but I've never seen a report on the forums of a StarGazer GPS puck failure.
I have the StarGazer and it's great. I bought mine in early 2011 and it has never once dropped the signal or given any strange behavior. My neighbor had an '08 Nautique with a 1st generation revision of StarGazer. His, on the same waterway, tended to behave as if it lost the signal and then just cut throttle sometimes. I'm wondering if they just added some additional logic in their software to account for brief losses of GPS signal
For a DIY install the StarGazer is super simple. No hole in the hull and it works great if you're not dealing with a significant current.
RideSteady seems pretty good but the display seems to leave a bit to me desired. It's just a single like LCD calculator type display. It would get the job done for interfacing with the speed control system but I'd imagine isn't quite as good at acting as a plain ole speedometer when the system is off. With PP it shows the speed pretty large on the multiline display. If you've got a boat with dual speedometers and will only be replacing one this wouldn't really be an issue but I replaced my speedometer with my PP gauge.
RideSteady does have a more robust looking throttle servo though. Not sure what it's like in real life as I've only seen it in pics. The PP servo is a bit cheap and flimsy looking but seems to do the job fine.
RideSteady did go up in price though at $895 (-$100 if you send in pics of the install). So, there's not really that much savings there vs. finding a sale price on PP. IIRC RideSteady used to be under $700.