It really depends on your needs, subject to availability, AA would offer discounted cash upgrades after booking and even at a lower price within the last 24 hours if your main priority is a larger seat. However, 2 x 50lbs ( 23kgs ) of baggage allowance + complimentary carry-on, complimentary seat selection and MCE would be a determining factor if AA is not extraordinarily expensive than Spirit.

I had rare occasions where I picked Frontier of Spirit on my US domestic travels but on most of the cases legacy carriers were asking 3 to 10 times what F9/NK offered which didn't justify the price difference even when purchasing add-ons with the low-cost-carrier.