I have cooked a many Select briskets, as Chef Jimmy stated, you're cooking it to fork tender anyway, so the grade does not matter as much as in a ribeye as far as tenderness. Now a few factors that will and / or can be noticed, Choice and higher grades do tend to cook faster than a Select, due to the internal marbling, you also have a better chance for moisture retention due to the higher levels of fat content as it breaks down during the cook. Stronger beef flavor is typically more evident the higher the grade of meat you cook as well. As Chef Jimmy also pointed out, there is a distinct difference in the chew of a properly cooked Prime or Waygu against a Choice or Select, a silky, melt in your mouth feel to it. True heaven in my eyes, but I'm a Texas boy born and raised on brisket so...
Another quick fact, the beef carcass is graded by 2 factors, the degree of marbling between the 12th and 13th ribs and maturity. So it is quite possible to be buying a brisket that came off a Select graded carcass, but be getting a Choice grade quality piece of meat. Of course you could also be buying a Choice grade (and priced) piece, but only be getting Select quality. This is why most people take a long hard look at the flat section of a brisket when purchasing and find the one with the most internal marbling and the most flexible flat section as well.