^^^^^^ What he said ^^^^^^^
The only time I'll buy a flat is when I'm making pastrami or corned beef.. I get my briskets at Costco like you, if they don't have prime then I go to another Costco. So to answer your question, since a flat is so thin, I'd go with prime because it give you more wiggle room so it won't dry out just my humble opinion..
Thanks
Greg
|