One of the nice things about an extended cab is the ability to store stuff in the back and have it locked up. And with EC rigs like my 08, the rear doors open 175 degrees which makes it real handy for loading gear in the rear. My truck has a rear 60/40 bench so you can even stow some stuff and still have some available seating if needed.
A longer wheelbase will help with the ride and there is also additional load carrying capacity along with more trailering capacity.
With your going off for training, what real need will you have for a POV outside of getting to a training base and coming back? Is the USMC going to give you any free time where you have a chance to use a POV? You might want to find out if that first 6 months will give you any free time at all.
Anyway, thanks for your service