This may not apply across the board but for Visa and MC cards issued by one large company, there was a difference, a big one, on rental car insurance.

One of the cards did not provide automatic collision damage waiver if the underlying rental was free (eg. with a certificate). However, the other card provided primary coverage on overseas rentals, meaning you did not have to file a claim with your insurance company and risk a rate increase.

I'd suggest FTalk readers check these matters out and choose carefully.