By what I read so far the Mustang unit gives a better read than the others by being able to run at speed and add variables.
The ability to cruise at 60 mph and get CO2 readings and etc. are a nice function.
They offer a "extra" for bikes @ 5k.
I saw the unit on Two Guys Garage and it looks nice, I haven't seen one in person yet, the closest one to me is in Atlanta.