Living in SFO with a UA hub it's hard to argue against UA. But, since I live in ORD and prefer DL over UA I'll make the case.

UA Pro's:

1. Hub
2. Frequency
3. Non-stops
4. E+ (believe me, it makes Y tolerable)


AA Pro's

1. Good elite product


DL Pro's

1. Good elite product (if you don't mind not being able to use your miles)
2. Transcon planes all have good IFE and wi-fi
3. Good hubs (MSP, DTW, ATL) if you don't mind the connections
4. Good option to CLT if you are not using US
5. More seats in F than UA (if UGs matter to you)


Personally, I have not had the best experience with UA. I may be expecting too much given they are in the middle of merger hell but I have been very underwhelmed. When they're fine they;re fine, but when there is a problem, they can be just absolutely awful. Plus, if you are not elite, get ready for the India call centers.