As to whether a 90mm would do a better job on 6x6 than a 80mm on a 6x7 - the answer is yes, but you would really be "looking for hairs on an egg" in that case.
Speaking more practically, however, I think you will find that it is much easier to find a high quality 80mm lens at a reasonable price than it is to find a high quality 90mm lens at a reasonable price. There were a lot more of the 80mm lenses made and sold.
In fact, you may find it easier and cheaper to find a pair of high quality lenses - 80mm and 50mm - then a single high quality 90mm lens.
Or you may get lucky and find all three.