If you're truly 50-50 AA UA then its a toss up. Only thing that might move the needle one way or another is where you are connecting to out of ORD. If you're connecting to airports that can maximize the lounge access on the other end besides just at home at ORD then go with that.
Concur that lounge access via co-brand CC is the best option. Perks on the cards might sway someone- for example the AA Citi Exec card has an annual rental car credit, monthly Grubhub credit, etc, that lowers the cost. UA Club card is notably weaker on non-airline perks but still holds value. Amex Plat and CSR for lounge access don't enter the conversation for ORD.