I don't know how my Visa (United Mileage Plus Platinum) or Amex cards (SPG, Rewards Gold Plus) compare to yours, but I'm looking at the same issue. I'll probably use the Visa for upcoming rentals in the UK because it is primary coverage overseas and clearly covers loss of use as well as damage. Well, OK -- another reason is that I'm getting double miles on car rentals through the Visa.

If you're using an Amex, another option is to sign up for the premium coverage. I forget the cost -- maybe $25 per rental? I considered doing that but will probably stick with the Visa.