Jeff, I've tried the mustard plus rub method quite a few times and like it.
I've also cooked briskets with just a rub.
It's hard to say if there's a great difference. Both methods, IMHO, produce good brisket.
This weekend, I'm going to inject a brisket and rub it with no mustard. I'll see how that comes out.
A good beef brisket would probably be quite tasty with just salt, pepper, and a little bit of garlic. That's what I do for tri-tips, and they come out great.