The Nova is by far the better motor. The Go is a good motor too and makes plenty of power, but they just don't last compared to the Novas.
That being said, everything goes out the window with a used motor. Who knows what the previous owner did with it. When it comes to used motors, only spend what you're willing to lose. Or if you want a solid backup motor, look at the Nova N21R. Great motor and cost less than $150 new.