Dan, it's so nice to "see" you over here!
I utterly hated sitting in Y on UA's p.s. planes--yes, technically it fit the definition of Econ + by coming in at the minimal 34", but, particularly with a single aisle, it felt sardine-like, and with nary an exit row seat to escape it, and I always considered it to be E+-. Of course, AA's LRTC Y seats on the transcons are even more cramped, but fortunately, I think I've always managed to snag an exit row when in Y. I would add that, leg room aside, I think AA's Y seats are less comfortable than UA's (though not as bad on the 767's as on the 777's); less padding or something. As for Biz, I REALLY LOVED row 9 (and would rate it higher than any seat in AA's biz class), but was less happy the one time I had to sit elsewhere. I do think AA's service, even with a bigger cabin, is better nowadays than UA's.But, as I believe one of the posters above has commented, ease of upgrading is definitely significant for some of us. As a 1K, I was initially always able to UG on UA, but that seemed to become tighter over time, and as a PremEx, I didn't think my chances were that good. I've had a good success rate at the equivalent level on AA, and that means a lot.
But the other reason I fly AA transcon is because AA's now my primary domestic carrier by virtue of their far better route structure in NY and it makes sense to fly them.