If you're looking at older trucks, get one with the diesel. The older diesels are simple, rugged, get great mileage and have plenty of power. Especially look for the Ford 7.3 Powerstroke, Dodge/Cummins 5.9 and GM Duramax 6.6 diesels.
If buying new or near new, the newer gas engines make plenty of power, are wayyyy less expensive to buy than the diesels, get descent mileage and are much less expensive to have repaired than the new diesels.
If you want an F150, get one with the HD Payload package, Max Tow package and Ecoboost engine. F150's without the HD Payload package run out of payload capacity quickly, especially if the truck is crew cab and 4wd.