In my experience I don't think it makes much difference, When I pick out my briskets I look for a blunt point and uniform thickness end to end. I will pick a choice over a select all things being equal.if the price is roughly the same. To be honest some of the best briskets I have cooked have been select. Grading on a brisket don't mean much because cows are graded between the 4th & 5th rib, the more the cow moves around the leaner the brisket is going to be(marbling) because that muscle is exercised more than the muscles where it is graded.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC