I'm a Ford man from way back and (in my honest experience) for pulling you DON'T want the V10 gas engine. It was designed for high revving and having fun, but not very good for heavy work and longevity. I drive an F250 with a 6.0 every day, a minimum of 150 miles a day. It pulls like a hoss, and with half a million mile on it the only "major" repair was having the injectors replaced a couple of years ago. The 7.3 was a good motor, but less power and worse fuel economy. The 6.0L can easily be beefed up with nothing more than a chip, and it'll pull Stone Mountain if you can hold traction.
6.0 first choice, 7.3 second choice, and if you are left with the only option being the V10, wait till you can find something else.