I installed the ridesteady system in my boat about three weeks ago and have been very happy with it, but you can't really compare the two. Ridesteady uses a paddle wheel while stargazer uses gps. I really think it all depends on what type of water you ride on (water with a current or none moving water). From my understanding gps reads land speed and of course the paddle wheel is based on your speed moving through the water. I have heard that pp has pretty good customer service, but I had no problem getting hold of justin at hydrophase when I had several questions on my install. I will give you my pros and cons for ridesteady since that is what I have, but can't really give you a comment on stargazer since I've never been on a boat that had it.
Ridesteady Pros: easy to use on a river with current, great customer service, less expensive then stargazer, detailed install instructions w/ pictures in manual and online, easy install, gauge is easy to read, don't have to deal with changing the settings depending on the amount of ballast you are using, quality hardware, reads water temperature, easy to adjust speed, has memory of the last set speed, gauge turns on with the turn of the key, does a good job maintaining speed.
Cons for Ridesteady: you need to drill a 2" hole in the bottom of your boat for paddle wheel, hydrophase is a new company (but perfect pass needed some competition), can't save multiple rider programs
There are probably other pros and cons I am missing, but the cons aren't really a big deal too me, plus I ride on a river and I'm very happy with what I've got.