We have memberships to both Sams & Costco. Like others have said, each seems to have a few items that the other does not. Overall, we find Costco to be cleaner, better lit, and better customer service.
Two things I especially like about Sams:
1) Business member hours. I can get in at 7am and be out in 20-30 mins, and the store is not very crowded. Our Costco is usually jam packed with shoppers, and most of them can't drive a cart to save their life.
2) Self checkout. Costco has self checkout, but only Sams has the handheld scan gun, and it works *much* faster and more accurately than the scanner mounted in the checkout machine at Costco. I don't even have to unload the whole basket, just point the gun & pull the trigger.
*** Bubba sez: A Bad Day Smokin' Beats a Good Day at the Office! - UDS, 26.75 OTS, EP-330, 18.5 WSM