First, there are no "official" towing requirements, standards, or tests. Each manufacturer decides the towing capacity of each of their vehicles. The standards that Ford uses can be different than Toyota (for example).
Second, how does anyone determine which is a "better" tow vehicle. Sure a bigger engine will allow you to accelerate more quickly, but won't make any a whole lot of difference when stopping. In fact the higher weight of the larger engine will tend to increase the stopping distance. During an emergency stop in vehicles with ABS systems, stopping distance is solely determined by the grip that the tires have on the pavement. Handling will also be determined by the grip of the tires on the road. Bigger vehicles generally have larger tires which gives more surface area and therefore better grip, but other that than there shouldn't be any difference.
I would expect that a minivan and a similar size pickup truck should handle about the same. You might be able to say that the van would be better because it might have a better weight balance due to the full body.
The only obvious difference between a truck and a unibody vehicle is that the truck might tolerate the towing loads better over the long term. However in the short term I don't see what the difference would be. No amount of experience, logic, physics, engineering, common sense, side by side testing, third party testing, first party testing will be discussed or apply.
My van has a GVW a bit over 8000#. The unibody is designed to handle this weight indefinitely. It should not matter a whole lot if the 8000# is contained inside the vehicle or is a combination of van and trailer.
FWIW, I tow a 22 cabin cruiser with a Sienna minivan (with tow package). I put airbags in the rear suspension. It accelerates acceptably and stops very nicely. Towing my boat is not any problem.
