I have both - but I find myself going & spending more at the BJ's.
What I have found out is that the BJ's has almost the same foods I buy at the costco, but in a smaller package, which works better for my family of 4.
Bag of organic apples is 5 lbs. instead of 10. Most of the produce and veggies are just a little bit smaller, therefore they don't go bad really b/c I'm not having to plow through the big big bags of items faster.
Organic ground beef is in a 3 pack, and 1 lb. each at BJ's. Costco has the meat in a 3 pack, it's $4 more but each pack is 1.35lbs each. A bit too much for what I use it for the 1lb. pack is perfect for us.
I like the pick your own chip section, better bread and roll variety, a deli, bigger milk and egg selection ( and organic milk is a 2 pack not 3) and I like that they have more of a selection of "name brand" items and not just the berkley-jensen stuff to choose from. Costco has kirkland which is great, but what I have found that in my costco sometimes an item is only available in the kirkland brand.
Anyway that's my .02!