A Daily Kos diarist reports that Costco treats its workers well (paying high hourly wages and providing benefits and getting low turnover in return), and pays only a modest salary to its CEO, while Walmart does the opposite. In addition to this distinction, there is the Costco pharmacy, which unlike many of its competitors, has a policy of not imposing their pharmacists personal views on a decision made between you and your doctor over what drugs (including the morning after pill) you need. Both chains have good prices, but one isn't being subsidized by Medicaid and Food Stamps. Apparently, Costco gains much of its advantage by not spending a lot of money on advertising.
A regular Costco membership costs $45 a year, and I've just about been convinced that it is worth doing, even though all of the locations are suburban.