Given a choice go diesel, you?re going to need the torque that a diesel offers when pulling a horse trailer (been there done that). But for under 15K the shape of the truck is going to be more important then engine type, really. A 2001 Dodge quad cab 2500 diesel with 75,000 miles, would sell for about 17k to 18k+ in my area. But in my area Dodges are not the popular and they tend to lose a lot of value quickly.
My dad used to be a long haul truck driver; he never had a problem with algae growing in his fuel tanks. Diesel won?t grow algae in it what happens is that as diesel breaks down if forms Asphaltenes. Asphaltenes are brown and slimy and resemble algae. Also algae need light to grow, how much light get through a fuel tank? None. Now stations that don?t move a lot of fuel can have a problem with asphaltenes. Just to be clear you can have bacteria and fungus growing in diesel fuel, but as long as you don?t have water in your fuel it won?t be a problem, just buy your fuel from a station that sells a lot and you won?t have a problem.
Now the price of diesel fuel, a diesel engine will have between 20?40% better fuel economy then a gasoline engine, so if gasoline is sell for $4.00 you could pay up to $4.80 for diesel and still come out the same if not better dollar wise.
My opinions are subject to change with new facts.