If we are very specifically talking about the AS Airbus Main Cabin Select or Premium Class (as it will be by summer) versus the UA Economy Plus, I'd take the AS Airbus route every time. Looking at today's flight schedule, UA is flying all 757-200s EWR-SFO with various configurations (likely different by summer). The AS Airbus configuration in MCS/PC offers more legroom, free seatback IFE (most of those UA planes have IFE, but would be $7 I think), and significantly improved food choices (although UA offers a "meal" - and we all laugh...). I personally prefer the exit row MCS/PC for flights over 3 hours so I can fully stretch out my legs, but those tray tables in row 3 are too good to pass up for flights under 3 hours. I've also decided the solution to the elbow pothole situation simple requires bringing a roll of painters tape to plug the hole - I wonder if the FAs would notice!?

BTW, welcome to FT OP!