We have memberships to both. Have thought about dropping one or the other, but I use them both enough to justify the dual membership. I agree with previous poster - it depends upon what you buy.
Even though mine are nearly next to each other, I use Sam's more. Both have petrol. Price is roughly the same (last I checked), but C's lines are unreal. I also shop two other Cs and one other SC in my area. In both cases the differing locales have different products that they carry. Most are the same, but a few are different.
Sam's- I use this one the most. Produce, meat (C's tastes/smells odd), seafood (better price, smaller packages), eggs (smaller package, brand I like), cheese (they use paper between slices, so you can get them apart), clothes, some furniture.
SCs produce is much the same from store to store. I also like that they have smaller 'bulk' packaging - i.e. 2 lbs. containers vs. 5 lbs containers. Entertainment electronics - TVs, Bose stereo, Onkyo receiver (SC got this set before C). C has newer electronics, but we wait a few months before buying. That way SC get them in, the price drops and they run specials on their credit card - no interest for 12, 18 or 24 months. Same basic return policy, but customer service has given me a little attitude from time to time.
Costco- Office electronics (laptops, servers, etc.), soft furnishings (pillows, blankets, area rugs - my Pottery Barn rugs were junk- Costco's seem better, so far) some few groceries, Costco brand laundry detergent & dishwasher detergent (Kirkland). Also the quality of produce seems to differ at C more. Couldn't tell you why.

Customer Service has given me a tiny bit of attitude, less than SC, but it's there.
Our local branch looks like they drop-kick the produce before attempting to sell it, but the 2 stores further out have nicer-looking stuff. I find that though Customer Service has a better attitude, the clientele at (our) C can be overbearing.
Oh, one last, weird thing - at C my car get more dents, dings and broken taillights.

Pretty sure this has something to do with the above.
