F350 for sure...I have an F250 and wish I had the F350....
As far as 6.4L versus 6.7L. IMHO, it depends on the year. Personally would stay away from the 2008s (the first year the 6.4 was used).
However, I would not hesitate buying a good 2010 6.4L....I own one. It has 60K miles on it and not one single problem so far.
2WD versus 4WD. I went with a 2WD and glad I did. In 5 years of hauling, never needed 4WD. Not having 4WD gives me about 400 pounds greater carrying capacity, lowers the overall height, and gives improved ride/handling due to the front suspension. Some even argue better mileage due to less weight and lower height but I am skeptical on this. Besides DW said no to the increased height of the 4WD, so a 2WD it was.....albeit with a limited slip differential.