There's also one other major difference between the two: the senior discount. Sam's Club offers a generous discount for people over 50: $20 for a Club membership or $60 for a Plus membership when joining as a new member. On the other hand, Costco doesn't have any existing discount for new senior members.
This is an interesting move on Costco's part, as the discount can easily be the deciding factor for a senior choosing between the two wholesale clubs. After all, how could you turn down a $20 basic membership? Especially when the lower-tier membership for Costco is already more expensive than that at Sam's Club.