from what I've read on the web
the 80/5.6 is a 6 element design, the 75/4 a 4 element. 6 element designs are deemed better.
I do have a 80/4 Schneider, a Fujinon EX 90/5.6 (and had a no-name Aston 75/3.5) I did a test and my order of preference turned out to be the Fujinon then the Schneider just ahead of the Astron (which was very surprising to me!) The Schneider cost me considerable $$$!. The Schneider has some nice user features like the aperture lever that alternates between your working ap and wide open. Good for focusing then stopping the lens down to the correct aperture without looking at it. It also can dis-engage click stops but I've never used that. The Fujinon can be turned when in the lensboard so that the highlighted aperture display faces you. Handy if the lens tightens and the ap scale doesn't face the front! I gave the Astron lens away with an enlarger but I was surprised how close to the others it was... I presumed it was similar to the bottom of a old style coke bottle