The world of warehouse retail has two top titans: Costco and Sam's Club. The chains have a lot in common. Indeed, they each trace their roots to Sol Price, who mentored a Costco cofounder and inspired Walmart's Sam Walton to launch his own version of the concept. Both clubs charge an annual membership fee ($65 at Costco, $50 at Sam's) that allows shoppers to peruse spartan aisles stacked with pallets of bulk-packaged goods at bargain prices. Another similarity is that each one carries products