I'm actually in the market right now for something similar. I would recommend gas unless your situation meets one of the following requirements: You plan to resell the truck in a couple years, you do a lot of heavy towing, or you drive a lot of miles. In these 3 situations the diesel makes sense. Otherwise you are paying a the higher purchase and maintenance costs without much return.
In most cases you can expect to pay ~$5k more for a 10 year old diesel truck. The truck has the same (if not more) wear on it than the equivalent gasser. In my situation, it would take ~10 years to recover the upfront cost of the diesel engine.
From a dependability stand point, I know more happy Ford owners than any other brand. Personally, I'm looking for a Dodge with the v10 motor. Good luck.