If you're going to fly less than 25,000 miles, UA will give you 5 miles per dollar spent. AA will give you actual flight miles which would be about 5,000 miles RT for a U.S. transcon. With UA you'd need to buy a $1,000 ticket for the same amount of miles. If upgrades are important to you and you're not going to fly enough for elite status, the miles you accrue will be needed (along with cash co-pays) and it's going to take a long time to build them up unless you supplement them with credit card spend on the appropriate airline's credit card.