I now have membership to both and like both. That said I've slight preference to Costco due to their business philosophy and lifetime guarantee. It gives me piece of mind and I buy stuff I normally might have second thoughts about. Both carry similar items but brands can differ slighty. Pricewise, Sams seems penny or two cheaper.
Costco is little further drive than Sams for me but I go out of my way to shop at Costco whenever possible since I want to reward good company. Oh I also own shares in Costco so as part-owner, I want to do my part in helping keep up the share price. Also Costco takes Amex which is my preferred method of payment.