I have tried them both extensively and I actually prefer Sprint Nav. It's actually made by TeleNav which is the most experienced company for mobile phone nav. On ther carriers(Verizon, etc) and platforms(iPhone) you have to pay for the same thing, while Sprint includes it as part of the everything plans. TeleNav is a more mature, well developed software although Google did well for their first attempt.
The voice on Sprint Nav is quite pleasant, whereas the one on Google Nav is very annoying.
Sprint Nav has better searching and browsing options. You can narrow your filter your searches by category and various other criteria including searching along your route and near other locations.
Google is starting to catch up to some of these features, but TeleNav still does it better.
The only advantage on Google is the availability of street view. If anything, that is an unnecessary distraction. I prefer a clean 3D graphic over some crappy, overexposed and outdated image from street view.
I haven't had technical problems with either one after many hours of use. I'm glad to have both options, but I'll stick to Sprint Nav.