Outside of signup bonuses, unless you're loyal to a particular airline or alliance (or 2 or more if you fly often), a general travel card with a large AF like Chase Sapphire Reserve, US Bank Altitude Reserve, or Citi Prestige will be more rewarding. A little lower on the list with significantly lower annual fees would be cards like Chase Sapphire Preferred or Citi Premier. Even for purchases on a particular airline, these general travel cards will often be more rewarding than airline-specific cards. Certainly run the math for your annual spending to see if a high AF card will outweigh the rewards from a lower AF card.
If you DO want to go all-in with one of those two airlines, Chase offers a multitude of United cards, and there are a lot of options for American with both Citi and Barclaycard. If obtaining elite status (or a higher one) is going to be important to you, possible lounge access other than Priority Pass with the very high AF cards, and/or you often check bags, then an airline-specific card may make more sense.
If those were my choices between united and American I would take American , you have four potential cards to choose from with American. Two personal and two biz. United has the option of the plethora of ur cards in addition to the United only card . If you are 4/24 you probably could only get one of the chase personal cards , as far as I know Citi and Barclays aren't as sensitive about 5/24 but not 100 percent sure about that.
In which case, definitely look at the various offerings from Citi, Barclaycard, and Chase. If you fly both AA and UA, it may even be worth getting both. Also, if your CSP is over a year old, do the math and see if upgrading to CSR makes sense. After the $300 travel credit, it's really only $150 vs $95 but the earnings are 50% more on travel and dining, it comes with more benefits, and if you were to redeem in the portal you get a 50% bonus rather than 25% bonus on URs.
