OK here I go into the merde'...From personal experience being an OO why do you think the majority of class 8 trucks are diesel, Power, for towing and hill climbing with heavy loads you really need torque to pull and climb. Flatland running with speedbump hills, not so much.
Diesel engines will with proper maintenance last 2 to 3 times longer than a gasoline engine however the maintenance and repair cost can be higher due to the fact that they are deisel. Gasoline engines are cheaper to operate and repair however with a shorter lifespan from the engine.
Simply put it is your choice and decision to make however I encourage you to listen to other people and their opinions and choices and also do plenty of research before making an investment into a tow vehicle.
Good Luck
OK Dodge Guy made a very valid point as to post 2010 deisel engines with the emissions package are,IMO, choked down.