It doesn't matter, I know some folks who like to use the brass long barrel nuts, just because they easier to get a wrench on, being longer and all. I always just use grade 8 hex nut and split washers on the race cars, this is not a fastner than normally gets stuck, I've never any trouble getting them off, even old engines. On my rebuilt engines I always get new stud and nut kit from Gordon at the B Hive, and they come with new white zinc plated studs and brass nut, so all my engine rebuilds get the brass nuts