If I had to choose one mill, I'd go with the dog trotter

Talking about fast, my dogs could run up to 59.8 miles per hour on it with a slight slant(killer spinner mode) Or keep it flat and have the dog run @ trotting mode. Raise the slat all the way up, turn the brakes on slightly and the dog will climb a hill. Turn him the opposite way and he's running down hill.
Adjust the adjustable shocks/springs to how heavy the dog is and the mill/dog will look like they're working in harmony.