The information should be inside the door of each truck. You can look at the sticker before you buy. You can order a truck and know when you order just what the carrying capacity will be.
Maximum cargo will vary quite a bit. It is essentially the difference between GVWR (a rating for maximum weight loaded) and empty weight.
Tire and wheel choices, suspension options determine the GVWR of the truck. This can vary somewhat in 3/4 ton models, more in 1-ton because of choice of dual vs single rear wheels, and much more in "1/2 ton." "1/2 ton" no longer has much meaning. High GVWR options for 150/1500 series trucks take them beyond where 250/2500 were twenty years ago. 1/2 ton once meant no heavier than 6000 pounds, but that number grew as EPA changed the CAFE rules defining what size truck is a passenger vehicle and what is a commercial vehicle, so now you find "1/2 ton" trucks with 8600 pound GVWR option, into the middle of what used to be Class 2 or "3/4 ton."
On the other side, cab size, trim levels, optional equipment can make for big variations in empty weight. A really large cab loaded with luxury can use up more than 1000 pounds of a pickup's GVWR, compared to a regular cab work truck with a plain bench seat and minimal electronics.
What you will get in a 250/2500 that you may not in a 150/1500 of similar cargo capacity is a drivetrain (engine, transmission, axles, cooling system) rated to be used at close to full capacity most of the time. Because this category is not included in the passenger vehicle CAFE, these trucks don't have to be engineered to run at low output during the CAFE test cycle, with higher power settings available relatively briefly either at high RPM, or in the case of turbocharged engines, at higher boost.
At a truck dealership (not just a car dealership selling pickups) there will be a knowledgeable fleet sales manager or fleet sales specialist who can help you order a truck to maximize capacity (it will look a lot like a work truck) or find one in regional inventory. A lot of sales people will not look beyond their own stock, especially if they are mostly selling only family passenger trucks.
Tom Test
Itasca Spirit 29B