My best cards are Sapphire preferred and Amex Gold, I gave both companies a call to see which would be more worth it for me, here are some of the (small) differences I was told of:
Length of rental to receive CDW:
SapphireL Up to 15 days
Amex Gold: Up to 30 days
How much is covered:
Sapphire: The amount of the car (excluding "luxury cars" -whatever that means, vans, and trucks)
Amex Gold: Cars up to 50,000 dollars (excluding trucks and vans)
There are many many differences, however only some are substantial to the average DDF user. If you know of any other substantial difference, or if you know the policy of other cards, please let us know!