i have both and use both. the 80 is much lighter and quieter and as mentioned many times can fully use VR lenses. the 90 feels more solidly built, is much heavier and at times the controls feel a little strange. the 80 is designed more like todays digital cameras, having two control wheels. if you are worried about batteries for the 80 you can get the small plastic grip for $15 at KEH and then use 4 AA's. but my two 123 batteries have lasted me more than 25 rolls in the 80 and still show more than 50% full, so while they do cost more they last longer. but the main difference is build. in nice clean weather the 80 is a great camera, but i would not want to expose it to long to heavy wind and dust or very dirty locations. the 90 doesnt worry so much there, but i usually grab my F5 in that case.
but the only thing that matters to me is the end result and to be honest, both are great cameras with great meters. I have a hard time telling the results apart from each other. with that in mind and the almost free cost, get both, try them out and keep one if you must. but I like having both of them as one always has velvia inside and the other tri-x. if you look hard you could get both for $100