From what I understand, smoother is better. Brushes are anything but smooth.
I'd recommend a belly pan and a custom aerodynamic topper. It can be made of plywood or coroplast if you like. Should go down at about a 15* angle.
Of course if you often haul tall loads that won't work too well.
When you make the belly pan, pull it straight forward, and wherever it ends up is how far down the air dam should be. Does that make sense?