I LOVE Costco! I go there for price and quality, and mostly for the produce. It's worth shopping there just for th samples!
Here's what I purchase there regularly (and bought yesterday):
Frozen Chicken Breast
Canned Chicken Breast
Canned Salmon
Gluten Free Crackers
Frozen Berries
Frozen Mango
Frozen Tuna Steaks
Colby/Cheddar Cheese (2 pounds for $4.50 yesterday)
Kalamata Olives (2 pounds for $7 yesterday)
Feta Cheese
Bleu Cheese
Field Greens (2 pounds of it there for what I paid for 10 oz at Kroger)
Fresh Spinach
Fresh Broccoli
Fresh Eggplant
Fresh Asparagus
Fresh Red/Yellow/Orange Bell Peppers
Grape Tomatoes
Oranges
Apples
Fresh Lean Ground Beef ($2.99/lb)
Stew Meat
Turkey Breast (not processed, just sliced from a whole breast)
Laundry Detergent
Cascade
Toilet Tissue (Kirkland's brand)
Eggs
Egg Beaters (Kirdland's brand)
Coffee (gourmet coffee...grind it on the way out of the store)
I also tried their Vanilla extract for the first time. Very good and much less than real extract at the grocery store.
I shop there once a month and get those items that I can't get elsewhere or that I know will be cheaper/higher quality there. Everything will be in a large quantity, so only purchase what you can store and reasonably use.
To me, there's no choice between Sam's and Costco. The quality I find at Costco far surpasses Sam's, and often our local grocers.