Is it better to use a card like Discover Gas, which pays 5% on all gas up to $1200, until I reach the $6500 on my AMEX Blue (but you need to reach $20 to redeem), or a regular gas Mastercard which pays 3% (like the ExxonMobil Mastercard) on their gas, but gives it off monthly on your statement , as long as you buy another tank of ExxonMobil?

Which card is a better filler car for AMEX for about three months on gas?