If buying used I wouldn't care at all if it was a 3/4 ton or 1 ton - they're so similar or in some cases the same that it doesn't matter. If there is a difference you can easily fill the gap with some aftermarket equipment or different tires.
Gas is a better deal used because they depreciate more. Also if you can live with the less popular configurations you can save some money as well. A regular cab or extended cab will be cheaper than a crew cab. Also, 2wd would be less than 4wd although that's a compromise I wouldn't be willing to live with.