I've seen a lot of discussion about Visa vs. Mastercard with regard to miles, points, and currency exchange rates but nothing but bits and pieces about which card is most widely accepted internationally.

1. Which card is most widely accepted internationally - Visa or Mastercard?

2. How about mainland China and Mexico?

3. Is there a reliable source of this information or do travelers just have to poll each other to find out?

4. Should a prudent traveler carry both?

Visa advertised throughout the Beijing Olympics that Visa was the ONLY card accepted there. I've been to places in Mexico that would only take one or the other but that was quite a while ago. So what's the current situation?