3/4 vs 1 ton?
Most of the difference IS in suspension, you can get the same engine in either truck but that suspension difference could be a big deal as it allows you a bigger payload. DOT (Department of Transportation) is starting to crack down on overloaded vehicles. So far, it's mostly been oilfield service trucks (usually mechanic trucks from what I hear) but the word is that personal vehicles are going to be next. Something to consider, but from my experience, it's tough to get a strait answer from the DOT.
Gas vs Diesel?
Pretty tough to argue the power of a diesel, it's nice to have when you need it. That being said, in my opinion a gas engine has more driveability. You don't need to allow it the same warm-up/cooldown period, noise, smell, ect. If the truck is going to be used around town it's something to think about. As far as cost goes, not sure. I would think that Taco has it pretty close. Diesel isn't as cheap relative to gas as it used to be and some of the milage numbers I've been hearing aren't that impressive. 7-10K for the diesel option and they ain't cheap to fix, but the resale value is there. Tough call, I'd think long and hard about what you're really going to use the truck for.