As a U.S. cardholder, I used to always use AMEX for international car rentals. Now I only use my Visa Signature card. AMEX used to provide U.S. card holders with PRIMARY insurance coverage interntionally and excess coverage domestically. Now I noticed in recent years that AMEX appears to have quietly dropped the primary coverage feature for international rentals. In contrast, U.S. Visa card holders appear to still have PRIMARY coverage outside the U.S.

Within the U.S., AMEX appears better because the coverage is up to 30 days while Visa is only 14. Both insurance is excess. Overseas, however, Visa appears to offer 30 day coverage that matches AMEX and becomes a PRIMARY insurer while AMEX is not.

Also worth checking exclusions, before traveling. Visa appears to exclude rentals in Israel, Jamaica, Ireland and No. Ireland. (Not sure about AMEX). AMEX and Visa may possibly also differ regarding which vehicles are excluded from coverage as exotic.

It is also worth checking what insurance extras (if any) that any special rates might offer that one might be able to access through a professional organization. I just about exclusively rent from Hertz and Avis using special rates my car insurance company negotiated because the rate includes a guarantee from Hertz and Avis that my liability is limited to $5,000 if I damage/total the rental car, they guarantee I will not be charged loss of use and they even throw in complimentary third party liability coverage that would be my greater concern if I were involved in an accident.