I think it boils down to 1% on everything and 5% on the specific item (gas or restaurant, depending on the card) and you can double it (i.e. 10%) if you take gift certificates instead.
You can see more here:
Gas card
Quote:Rewards: With this Cashback Bonus program, you will earn a cash award on all qualified purchases, up to 5% of each gas purchase and up to 1% of each other purchase, based on your annual level and type of purchases, with your first $1500 in purchases accruing awards at 5% or 1% respectively. You will not earn a Cashback Bonus award on annual gas purchases in excess of $25,000. Transactions must be sent to us under merchant category codes 5541 and 5542 to qualify as a gas purchase for purposes of calculating your Cashback Bonus award. For purchases made at select warehouse clubs, discount stores and their affiliates, you will accrue a cash award at a rate of .25%. Get up to 10% Cashback Bonus award when you exchange your total cash award for up to double the amount in gift cards or certificates from our Cashback Bonus Partners. Award is redeemable in $20 increments. Other restrictions apply. You will receive complete Cashback Bonus Terms and Conditions with your Card.
That's the gas one and the restaurant one is pretty much the same, except for restaurant merchant codes.
I was looking at the shell card (mastercard) and it gives 5% back on gas and 1% on everything else.
I'll prolly get the default discover and the shell card.