Before, if you brush it after, when you eat all that nasty junk that builds up in your mouth while your asleep you're eating it. That's the whole point to brushing your teeth in the morning. If you eat breakfast first then you may as well not even brush your teeth in the morning. If you're trying to leave the house with minty breath by some bubble gum for when your done with your meals or keep a bowl of peppermints around.
A brush before breakfast is absolutely important for I don't wanna eat my waste-over-night. Whether I brush after breakfast again or not depends on how I'm feeling that day. But no matter how, I would suggest you to gargle your mouth with salt water immediately after waking up.
Personally I brush my teeth before breakfast, because I want to enjoy my breakfast with a fresh mouth and not a mouth that taste like sleep. Then when I'm done I use Listerine so the smell of breakfast won't haunt me later in the day.