The N75 has slightly more metering segments than the N80 (25 vs 10). The N80 has a slightly faster max shutter (1/4000 vs. 1/2000) The N80 has slightly faster max frame advance. The N80 allows for Manual and Auto ISO for film; Auto ISO only in N75 The N75 has some special program modes; the N80 does not have them. The N80 has a slightly faster max flash sync: 1/125 vs 1/90 (typically you will end up with 1/60s unless you are in bright light). The N80 uses CR123A batteries; the N75 uses CR2 batteries. The N80 has ondemand grid lines; the N75 does not. The N80 uses the old style cable remote release, the N75 use an IR remote release.
At the time of their release, the N80 was the top of the line amateur camera; the N75 was mid line amateur camera.
The F100 was the entry level pro camera and the F5 the top of the line pro camera.
Neither the N75 nor N80 will meter with manual focus, non-chipped lenses. (The AI-P lenses do have a chip.)
Both are compatible with AF, AF-G, AF-D, AF-I and AF-S lenses.
The N6006 is a good camera, but the metering is older and the AF is not as good; a generation older than the N90/N90s/F90/F90X and N70/F70 AF and metering modules. I believe it will meter with AF and MF lenses; It does not use distance information in exposure calcs or flash exposrue calcs.
The N70 was before the N80; solid build, little brother to the N90; funky user interface due to the built in flash's capabilities. I acutally liked my N70 more than the N80 for different reasons (ability to meter with manual lenses, for example).