I recently flew United's Premium Plus IAH-LIM-IAH and had a chance to compare it to pre-pandemic American's Premium Economy MIA-BUE-MIA.
Both UA and AA use identical seats with identical pitch, but UA edges out AA on service.
While they both use plastic cups for the initial drink service, the dinner tray comes with a proper wine glass on UA, whereas AA sticks with plastic.
The quality of meals was about equal on both UA and AA. However, UA flight attendants not only made eye contact but were genuinely friendly and helpful. AA flight attendants couldn't be bothered and were about as disinterested and disengaged as you could possibly imagine.
UA amenity kits were better and more well-stocked than AA's, and the Saks Fifth Avenue pillows and blankets were superior to American's offering.
All in all, there wasn't a huge difference between the two, but UA's attention to the little details, and genuinely more friendly flight attendants made a huge difference in the overall impression.
Look, it's been swell, but the swelling's gone down.