I would definitely choose the diesel over gas. I have had both and hands down you will be much more satisfied with the diesel.
On 2WD vs 4WD, you will find especially when empty that the 2 WD will be an issue anytime you get off the pavement especially with any kind of incline. I am not talking off roading, I am referring to loose gravel driveways that have a little incline, wet grass, etc. Not as big of an issue when you have the trailer on the back, but definitely an issue when empty.
It will also help your resale in most areas if you need to trade.
Just my thoughts. Godspeed, Brian
PS, just noticed that I have not updated my profile, I now have an F350 SRW 4x4 pulling a Heartland Sundance 5ht Wheel.