of course there's the big argument that republicans shop at sam's and costco is more democrats... but we didn't want a political argument did we?
I belong to both Costco & Sam's... (I used to be an executive member @ costco.. this year that didn't make sense so I decided to get the regular and w/ that savings we bought the costco membership)
Outside of products...there are many subtle differences between Sam's and Costco. I personally much prefer Costco but Sam's is closer. The lighting in Costco is brighter and it just feels cleaner. Sam's is dimly lit. Costco employees are very well paid, much more than Sam's employees. well paid employees==helpful employees. and its nice to support organizations that don't try to use their size to belittle everyone in their chain (as wal-mart/sam's is well known for doing).
sam's web site sucks (if you're looking at certain products, they don't even list model numbers). costco's is great.
costco has a lot of fringe benefits too.. sam's tends to do the same, but if you look at history most of the more innovative things were started at costco first then copied by sam's (even the $1.50 hotdog+drink was copied...). they have a lot of services that I'm not sure if sam's has copied yet... i've gotten great deals on various things (saved a ton on a car loan...)..
...now to the products. its somewhat hard to compare the two apples to apples, unless the stores are very close geographically... both sam's and costco's carry different products at different stores (or package them in different sizes even). I find that Sam's usually has a wider selection of the same products. For instance, they might have 4 different types of dishwashing detergent whereas Costco might have one major national brand and then their own usually very good Kirkland Signature brand. I prefer the Kirkland Signature house brand over Sam's house brand. Sometimes Kirkland Signature branded stuff is better than the major national branded stuff, and its sometimes priced accordingly (some of the KS stuff is priced higher than the major brand stuff).
I prefer Costco meats, though a lot of the pre-packaged stuff is identical.
Costco seems to carry more "luxury" items, and stuff I might not have thought about buying unless I had seen it and thought it was a good deal. Also, they put out a Passport savings coupon book once or twice a year, and always have coupons of some sort. i've gotten very very good deals w/ these coupons (buy one get one free pork loins, detergent, etc.. $10 off this, $5 off that... etc). I haven't seen any coupons from Sam's.
Someone said they prefer the Sam's wine selection... costco has more higher end stuff, but they have a decent selection of average/inexpensive wines too. I don't feel strongly one way or the other, but i tend to buy more wine when i'm @ costco than i do at sam's... so maybe i prefer costco. also, costco is the #1 seller of wine in the country.. so that must say something too. (or maybe that relates to the fact that sam's are typically in more rural places where they might not be into wine as much). from the bottles they do have in common (the popular champagnes, etc), costco's prices are lower than sam's...
though lately i've been finding better deals at real wine stores than i have at costco/sams.