re: Sam's or Costco regarding better quality meat?

Excellent isn't a USDA grade. Some companies have special programs. Some of that is just marketing, some of those are filed with the USDA and have Hard and Fast standards. For example:

Certified Angus Beef (CAB)
Sterling Silver

Those require the packers use certain breeds, cows younger than a certain age, certain marbling scores within the Choice Grade (upper 2/3 choice). Typically, you'll get a really good piece of meet from those programs. Not Prime, but as close as you can get to it without Prime prices.

I've never heard of Excellent though.