[Michael]

I primarily use AMEX Blue, with Chase Freedom Visa as a backup.

I only have two Mastercards, the Citi Exxon/Mobil Mastercard and the Citi Driver's Edge Mastercard. Neither gets much use, but if I were using one as a 3rd card, which would be better?

The Citi Driver's Edge gives better rewards, 3% on EDP and 1% on all else. Plus they allow you to sign up for the mileage program, where you get $1 for every $1000 miles driven. You can redeem your rebate (Max $1000 annually) for a new car or car repairs when you reach at least $25.

The Exxon/Mobil Mastercard pays 3% on ExxonMobil gas and 1% on everything else. Rebates are credited to your next Exxon/Mobil purchase. The payback, however small, is quick and uncomplicated.

The Citi Driver's Edge is clearly the better rebate card, when you are using it as your primary, or even backup to AMEX, card. You do have to send in all the mileage from your next service receipt to verify the miles. When you are ready to redeem for a car, you buy the car, and then send them a copy, and then they send you a check. So, there is some paperwork involved, plus the rebate-building on this card is more of a long-term deal.

Exxon/Mobil is far less complicated. You don't need to request or send in anything. The payback is simple, and automatically shows on the next statement where you make an ExxonMobil purchase.
I use a lot of ExxonMobil gas, no matter what credit card.

I have already asked and you CANNOT combine these cards, due to the fact that Exxon/Mobil is "on a different platform" (whatever that means), according to Citibank. I guess that this has something to do with the fact that they bought the card portfolio from ***ociates Bank.

The Mastercard (whichever) would not get much use, except for when I am under the $6500 AMEX tier.

I do not want another card. I just want some input between these two cards.