We have both here, within two blocks of each other.....I'll never darken the doors of Costco ever since the Northridge earthquake collapsed one entire wall and they were closed for six months -- but would not prorate the renewal.
Our Sam's is great because it's never at all busy; in and out without getting that "dirty" feel we get at their parent, WalMart.
Bonus - snack bar's huge Nathan's hot dog/drink combo for $1.50 is about the cheapest lunch around.
My wife and I shop Sam's about once every two weeks mostly for meats. I like their fresh ground beef and their pot roasts. Their rib eye steaks are the best in town. We also buy rice and flour in bulk about every six months and transfer to 5 gal. food grade plastic buckets for storage.
I have never had a long wait at check out nor when leaving for the door checker to mark our receipt.