FWIW I found that the difference is constantly changing. If it means a lot to you, consider waiting or just consider the Tango fare is a bargain and go for it. If you choose not to pay the fee for the advance seat selection, you might still be able to do it on web check in. If it is oversold, you will be subject to an involuntary bump if you have no assignment, giving you an MCO and full AP miles on the J class on the next flight.