I use both as well. Here's a bit of a breakdown on what I buy where:
Winco:
Greens
Lemons
Broccoli
Carrots (2 lb bag, $0.88)
*Oats
*Flour
*Sugar
*Baking power
*Baking soda
*Beans and lentils
*Spices
Yogurt for starter
Frozen juice concentrate
Cereal
Costco
--------
Butter
Cream
Bacon
String cheese for kids lunch
Dried blueberries
Sliced turkey breast
Over the counter meds (allergy meds, tylenol, naprosyn, vitamins)
Olive oil
Chocolate chips
Frozen berries
Frozen vegetables
Bottom line is that I use each for different things. The membership is definitely cost effective for us.