Hello Craig,
Twin axle caravans are always difficult to manoeuvre by hand as you probably know. The use of a motorised unit to help is usually very welcome, but as you point out in your question there are two generic types, the hitch and chassis mounted units.
It is a physical fact that the maximum pulling or pushing force (drawbar) any wheel driven mover can develop is equal to the force pushing the driven wheel to the ground, and the friction developed with the ground.
Hitch-mounted movers only have their own weight plus the nose weight of the caravan, which may be up to 150Kg Max. This is the maximum drawbar force the unit can apply.
Alternatively Chassis-mounted drives have approximately half the weight of the caravan on each driven wheel, so assuming a 1 tonne van, you will have about 500Kg down force on each wheel, and potentially that much drawbar power from each wheel.
The limited drawbar effect of the hitch-mounted devices can become a problem on inclines, where the weight of the caravan can quickly exceed the grip of the mover, and the caravan can run away. As a simple guide, if you could not stop the caravan on you own, there is a fair chance that the mover would also loose control. The problem is even worse when the surface becomes slippy.
You have a much greater margin of safety with a chassis mounted mover, plus the larger diameter of the driven wheel allows it to negotiate rougher terrain, or loose surfaces (Gravel) better
A hitch-mounted mover may be perfectly adequate with the ground conditions at home, but what happens when you are away? There greater capability of the chassis mounted unit can give you confidence. Also it is always ready to be used, where as a hitch-mounted unit usually has to be removed whilst travelling.
By far the majority of correspondents to this forum indicate that chassis mounted movers are preferred to hitch-mounted units. In many case owner of hitch-mounted units report replacing them with chassis mounted drives. When you consider that majority of caravans are single axle, then the challenges with a twin are somewhat bigger.