They both are good but have flaws
I have used Sprint Nav for a few years, and it always works great, but getting it up and running has been a pain every time, wether or not my phone wont access the GPS right away or the application crashes. I have to usually allow about 5 minutes to get it started, type the address and get it ready and synced with the GPS signal. *assuming i do not have to pull the battery* But Sprint Nav has always been very accurate, the turn by turn is awesome, and its very easy to look at.
Google nav on the other hand is awesome, it starts much faster. But my favorite feature on my Android handset is that I can type in a place, like the vet for instance, and if the address is in the google search results, then you can click it, then open it up in Google Nav and it will direct you there from your current location. Which is something that I do not know if Sprint Nav can do. I have tried before, with no luck. So thats the main reason i voted for Google nav. However it generally gets you to the area you want, but not directly there.
So sprint nav is harder to use but more accurate, but google nav is easier, faster, has more features, but does not quite get you to the front door.