Truck
There is a plus and minus to the long bed.
The plus: With a long bed truck, you won't need a slider hitch to provide extra clearence between the cab and trailer in tight turns.
The minus: The truck does get rather long, somewhere in the neighborhood of 23'. You will have problems parking in the average Wally world, K-Mart or mall.
Personnally, I would go with the long bed because I have seen the damage a tight turn can do. You won't have to worry about getting out and sliding the hitch. Repairing the truck and trailer will cost far more than the cost of the long bed truck.
As far as the engine, you have to know how heavy your trailer is. The V-10 is not known to be gas friendly. If you are not towing a LOT, and the smaller engine will do the job, you will get better mileage when not towing. Go to a Ford dealer and get the tow ratings on each engine, in that truck, and see what fits your needs best. You are wise to avoid the 6.0 diesel.