I liked the DBU much better as it had the re-active reducer and made the base lock up , and not move , & more user friendly , plus DBU didnt have the lift cycle that DBC can have Makes a big difference in the re-finish market .
But DBC also has a hardner ( I think DX59 ) you can add to the base to make it lock up like DBU .
I have been shooting DBC for the past 3 years as PPG is slowly fazing out DBU .

Sprayed alot of DAU 75 myself , I was using it as a base clear many years ago over Delstar , I think thats why I went to base clear so well when it was introduce , I hated Lacquer and hated the look , metallics always looked flat , unlike an acrylic enamel in my opinion .
DAU82 was a very nice clear but was a 3 coat clear in its day , unlike the high solid 2 coat clears of today .
I hated the F'ers that laid down lacquer then shot DAU75 over it , what a mess if you had to get back into it when it was damaged !

I might add that PPG's Clears are formulated for Heat make-up booths , but they can be air cured , wont hurt them , but you better make sure you have air movement for a long period or they like to solvent pop . FWIW .