Bulk-buying, bargain-hunting shoppers have made the members-only discount store concept wildly successful. Costco Wholesale (COST 0.88%) and BJ's Wholesale Club (BJ 4.26%) are translating that popularity into strong growth and record profitability. At the time of this writing, shares of both companies are at an all-time high with Costco up 44% this year and BJ's up a similarly impressive 40%.Should you buy Costco or BJ's Wholesale stock now? Here's what you need to know. The case for BJ's Wholesale ClubDesp ...