IIRC Costco takes Amex (and not Discover) and Sam's Club takes Discover (and not Amex). It's been a while since I have darkened the door of a Sam's Club, but at

IIRC Costco takes Amex (and not Discover) and Sam's Club takes Discover (and not Amex). It's been a while since I have darkened the door of a Sam's Club, but at the time I thought that was a pretty big win for Costco...