In our area Costco gasoline and diesel is priced at $0.05 per gallon cheaper than cheapest local area gas station. They have a list of stations that are checked every morning and prices adjusted.
As for value and quantities, I don't see much difference between Sam's Club and Costco, especially in our area. There are three Costco Stores within 10 miles of our home, there is one Sam's Club about 15 miles from our home. Two of the Costco Stores have gasoline, the Sam's Club doesn't.
The Sam's Club had a really good deal on tires for the DW's Explorer, Sam's didn't take credit cards for tires or batteries, Costco took AE. Now I think Sam's takes MC, cash or debit. I don't used the debit card because once I forgot to tell the DW I did and it applies only to her account.
Costco and Sam's tend to sell the same items but made by different manufacturers, some things like beer and spirits overlap, Wine not so much. Sam's tends to carry stuff more prevalent in the mid-west and east, Costco tends to carry stuff from the West.
When we did our 2011, 13k miles trip east, we asked for a list of stores from Sam's. We were told that it was on line. Costco has a brochure with all the stores and what features each has. Costco's list is published twice a year.
Without a guide to the stores, it was harder to find Sam's Club store and even harder to find the ones with gasoline, not listed on line at that time. We do have a couple of regular stops where we purchase Walmart gasoline, and there isn't a Costco close by.
I haven't been to too many Sam's Clubs so I can't comment on parking lots but the one near us is a total pain and my DD is a Suzi. It is worse than the worse Trader Joes Market and their lots are always a topic of conversation when talking about grocery shopping.
I pay for my brothers Costco membership, he pays for my Sam's Club membership, he buys more gas from Sam's Club because it is closer but he lives in a very rural area and I in the big city
So all in all, I prefer Costco even when fueling the Winnie and the towed.