1 ton. Unless you're using a portable or you're a padder I wouldn't consider anything else. Especially if you plan on carrying water. The difference in price isn't that much and you'll save on brake jobs in the long run.
I prefer the 3/4 ton vans only for the ride. Brake jobs are easy to do so I wouldn't buy the 1 ton for that reason. I bought Matts equipment and was concerned when I found out how heavy it was, but it rides remarkably well with the machine and the water tank full.
One ton extended....I bought the whole package from Butler. For some reason, they seem to have the best package w/o a lot of hassle. Hate buying trucks....the Butler GMC is well engineered and thought out. They have the suspension package/tyranny ratios figured out. Also, they have a different sway bar or something on the front end that is a minor option.....but the front end holds alignment better and the tires seem to wear better compare to last one ton I bought myself in '03.