I personally own a N90s. Great camera. Even though its hard to tell the differences between the N90 and N90s, the N90s has better metering. The vertical shutter release on the battery pack (MB-10, which is used on both cameras) only works on the N90s.
The N90 and N90s have stronger bodies then the N80. The N80 is more of a consumer camera, while the N90(s) are used by many pros as backups.
One big difference between the newer N80 and the older N90(s) is that the N80 is fully compatable with newer "G" lenses and lenses with Vibration Reduction. The N90(s) can use "G" lenses but only shutter priority and auto modes, and VR will not work at all.
I believe the N80 also has an improved matrix metering system similar to that of the D70 and F100.
K