Furman, I don't have a problem with Duke's, I just prefer Hellman's. When I was a kid, we moved to NY for a few years, and then we moved back to SC. Hellman's was what we got up there, and when we moved back, Duke's was what we got because we didn't have a lot of money, or at least that was my opinion. I thought Duke's was the cheap, generic mayo. I don't have a problem with either, but prefer Hellman's. I know this will be horrible for some to hear, but depending on the meat on a sandwich, I sometimes will get crazy and put a little Miracle Whip on it, but I don't consider Miracle Whip to be real mayo.
As a side note, when I was in school in NY, I was in K-2nd, so around 1973-1975 or so, I loved the cafeteria's peanut butter and jelly. I couldn't figure out why it was different than at home, but finally found out they put mayo on them. It sounds nasty, and I haven't tried it like that in years, but I liked it back then.
As far as mustard, it depends on what I'm in the mood for. Yellow and brown are totally different tastes, so it depends on mood and what it's going on.