I suggest that you consider the tow vehicle in the same way I consider my tools. It is always better to have more capacity than we think we might require. It is never a good idea to use any tool to the limits of its capacities on a regular basis.
A tow vehicle may have a manufacturer's rating for pulling 7000 lbs., or it might have a rated capacity for 12,000 lbs. The vehicle rated for 7000 will work at or near rated capacity nearly all of the time, and may easily be overcome by hills, mountains, high altitudes, or other challenges. But the vehicle rated for 12,000 is likely to pull the same load with ease under nearly any conditions, and with far less strain on the engine, transmission, cooling system, etc.
More than you think you may need is always a good thing, in my opinion. Beyond that, all I can add is that vehicles with a longer wheel base and wider track are generally more stable under any and all conditions.