Personally, I love Costco and find their product quality and employees to be better than Walmart / Sam's Club. The number of Costco members keeps increasing year over year so clearly they're doing something right [1].
After shopping at both, the quality difference is non-existent. The variety of products at Costco is better but my staples inside bulk meats, veg, and fruits and not housewares or dry goods so its a wash.