No good comparison in weight hauling capacity from a 3500 series to a true (not pickup) 450/4500 series. So even if you are a die hard GM guy GM is not an option for you unless you want to sacrifice big-time payload. Those mason dumps are heavy! Figure each truck will weigh about 10K unloaded. So you get either 3300 or 6000 pounds of payload. That's a no-brainer. In fact if I were going to get a little mason dump I'd go right to the 550/5500. Truck costs a little more and weighs a little more but then you can get a payload of closer to 8000 pounds. You can almost do real work with that.
On the gas vs. diesel, these days that's tough. Ford still uses the Triton V10, a tried and true engine. With diesel emissions garbage these days, the gas engine will almost certainly give you less headaches and cost you less in repairs. That said, diesel will give you better mileage so you will get fuel less often. Not saying you will save money though. A diesel will really shine when hauling heavy and towing at the same time. That's what these motors are made to do. I'd drive both and think long and hard if you are going with Ford.
The other guys did well to bring up the Ram, or Dodge, or whatever-they-call-themselves this week 4500 series. I would give them strong consideration. No gas option with Ram, as everybody knows they use the Cummins B series engine, of which there are probably about 100,000,000 of them in use around the world in countless applications. They also use an Aisin-sourced transmission. Aisin builds transmissions for Isuzu and other Japanese trucks, and have a great reputation.
So I would do your homework. In my opinion brand loyalty has no place in buying a truck for commercial use. Keep an open mind and while it's always good to keep others opinions in mind, they are all worth what they cost you (which of course is nothing), find the truck that you like and will suit the needs of your business.