There are two different rebates going on here.
1. Costco offers the difference between the two memberships if you are not satisfied. I am not sure if it is just for the first year or if it is for ANY year. Ours has always been worth it to us.
2. Costco gives a rebate based on purchases. This is only good at Costco, but can be used a number of ways: cash at service desk, pay for membership, or pay for purchases.
In addition, you can apply for the AmEx card with the Costco Executive membership. There are different percentages you earn based on the types of purchases. You earn 1-3% on each category, including all other purchases (not related to Costco at all, which is a big deal to us). This is a separate check from AmEx and it can be cashed at the Costco service desk or used to make purchases at Costco. I've done both of those. I think there are other options, too, that we have not done.
I particularly appreciate the AmEx card because it has my picture on it and serves multiple functions, saving me time and energy and space! I walk many places in our community and I can just carry my AmEx card. Even when I do drive beyond our community, I use my AmEx card over my Visa card because I dislike having to dig out my driver's license from certain wallets/purses. We don't have spending issues with credit cards, so this works well for us. We pay off the AmEx bill every month and earn a nice rebate annually.