Sam's club has "passes" for non-members, but they charge an additional 10%.
The Costco owner and founder, makes, I believe, only $350,000 per year(I just recently saw this on TV). Costco, every once in a while, has incredible deals where the goal is specifically not to make money, but to give back to their customers. An example is the Costco HID. They sold them for exactly one dollar over their shipped price, which is absolutely unheard of anywhere. We all know that Harbor Freight sells stuff at very reasonable prices, but they're selling the same HID spotlight for something around $130. This past Christmas, I got two pair of some really nice Ugg style slippers (leather and lined with lambs wool) from Costco and they were only $12 each. They are very similar to Ugg branded slippers, which sell for around $60. In fact, I actually like the Costco slippers much better than the Uggs, because they're lower to the grow and are more comfortable to walk in!
I do know that Sam's club has a kick *** warranty policy on their electronics items. If it breaks within the warranty period, you can just bring it back in, and if they can't fix it, they'll give you a new one! That is with the standard warranty that comes with most items. It also applies to their extended warranties, which are incredibly cheap compared to other places. For Christmas, I bought my dad an Epson Picture Mate Deluxe for $179 their. The extended warranty, which starts *after* the 1 year manufacturer warranty, was only $40 for three years. So, if the thing should stop working in the next 4 years, they told me that all I have to do is bring it back and if they can't fix it, they'll give a brand new one. I was also under the impression that they would most likely just give a new one on the spot, because the way she led me to believe was that if they can't fix it right then, it'll be replaced.
They both have lots of tasty food samples, too! Oh yeah, here's one big difference, in my opinion: Costco's pizza is much better than Sam's Club Pizza (the hot and free one). It might just be the location I went to, or the time I bought it, but they just seemed to be doing something wrong. Maybe the dough wasn't cooked enough, or was too sweet, or both...I don't know.
Overall, I can't really say that I like one more than other, from a buyers perspective.