Just remember that you have to consider your driving style, and where you drive when making this decision also. Most of the boxes that sit on the floor of the toad and the ones that attach to the hitch with no connection to the coach are inertia based systems. That means that unless you are coasting freely or accelerating, the toad brakes will be on. In particular, on a diesel rig with an exhaust or engine brake, when you're going down that long, long hill (think west coast mountains - 6 % grade next 5 miles....), your toad brakes are on a little bit all the time.
This is an absolutely critical statement. And, its not just mountains. Unless you turn of the engine brake, every time you take your foot off the accelerator, after a short delay, the engine brake kicks in. Your Allegro Bus has engine (Jake) brakes. You need to purchase a system that provides proportional braking, activated by the coach brakes.
One of the best is M & G Engineering's product. I've used it on two toads. My father also uses it. Here's a review of M&G's system by Mark Quasius M & G Tow Brake Installation