We used to switch back and forth between the two: One year at Costco, the next year at Sam's. Due to their warranty policy (the warranty is only good as long as you are a member) we stopped switching and stuck with Costco since we needed to extend a particular warranty. However, this year I think we are going back to Sam's.
We have four Costcos and two Sam's near us and there does not appear to be any set standard for either. The stores carry different products and have different prices (I purchased a clock radio that was $10 cheaper at one Costco than the one we normally frequent.)
Customer service also varies. Unfortunately, the Costco we normally frequent has the worst (the major reason we are considering going back to Sam's this year). The checkers just sit and talk with each other while the lines back up.
The best thing to do is to visit the stores near you and decide based on your impressions.