re: Grade of brisket?

It does matter. Select has less intramuscular fat (marbling). Long, slow cooking, will cause it to dry out easier as a result. You may be able to get away with it if you were chipping it up, but it’s definitely less forgiving. It’s cheaper for a reason. Grading meat is voluntary (all meat has to be USDA inspected, but grading is not mandatory). The higher the grade, the more marbling.
Breed of cattle also matters. Black angus has bloodlines connected to wagyu breed. That’s why you pay more for CAB (certified Angus beef). This is not to be confused with red angus, which sometimes is advertised as “angus.” It’s a sleight of hand. TMI, but the bottom line is: quality in / quality out & garbage in / garbage out