I don't have a Costco very close to me, and have (2) Sam's Club warehouses within 20 miles, so I buy Sam's Club Prime Brisket, and have absolutely noticed a better outcome when using their Prime grade briskets versus any other choice. The brisket doesn't dry out, and at 3.49/lb, I think it's a fair price and good selection usually.
That being said, I did pop in to a Costco when I was traveling on business, and their Prime brisket selection was just plain beautiful. I'm not sure you can go wrong either way.