The primus and prodigy units, is there any benefit with a small trailer with a consistent loaded weight?

This would be dependant on the travel route the tower would be taking.  If the tower will be towing a small consistent load over relatively flat terrain, than an 83501 (Time type) controller would be more than sufficient.  However, should they be traveling on varying degrees of incline and tow often, they would be better off with an inertia activated style (Primus / Prodigy).  These styles self adjust to compensate for steeper inclines that require more breaking power.  If you had a regular brake control, they would likely need to use the override button to increase the breaking power executed when traveling down the steeper inclines.