I can comment a bit but I will add that I haven't used the Spartan Board. I have however used the gyro that the Spartan Board uses (
http://www.analog.com/media/en/techn.../ADXRS453.pdf). It's not any more or less accurate than the NavX IMU.
I'll also say that our team has used the NavX for a couple of seasons now and we're madly in love with it. So much so that we wrote the LabVIEW libraries for it. We're also doing some neat things with the NavX micro now too.
They are both great products and each have their uses (the locking PWM pins are very compelling). That being said, using an IMU or Gyro really comes down to your code for the most part.
If you're having issues bouncing over defenses then it's possible you were trying to poll data too quickly or not filtering out anomolous data or didn't do enough testing. It's likely not the hardware.