Walmart and Costco have outperformed the S&P 500 over the past five years, but onestands out as the better long-term buy today.People have to spend their money somewhere, and many are choosing budget-friendly favorites, Walmart (WMT 0.96%) and Costco (COST 0.81%). These retail giants generate almost $1 trillion in combined revenue each year, but they have different business models.Costco focuses on membership warehouses and has 914 locations worldwide . You don't need a membership to enter one of Walmart's ...