thats a function of the issuing bank (e.g. chase, citi, etc), not of the card
ime, from living near london, and this past month's holiday to turkey and greece, visa is overwhelmingly more accepted than amex. true everywhere in america, and the 30+ countries around the world ive visited
Which is why amex kicks ass (by giving you cards directly serviced by them)
not that I have ever had trouble with my visa issuer (us bank)...I've called them from the airport while waiting for a plane to take me off to a bunch of different countries over 3-4 weeks and other than hearing the intercom in the background and saying "you're calling from the airport aren't you" it all went without a hitch.