Jim, just get the half ton now. We got our half ton (11,200 pound towing limit) when we were at 3500 pounds loaded. It was a beautiful experience....truly stress free towing. We moved up to 7k pounds after two years and didn't have to change the truck. Saved us many thousands and the hassle of changing. My personal rule is not to exceed 60% of max towing. More margin than most recommend.
I don't know what payload numbers are like between the mids and the half tons? If it buys you a few hundred pounds more (sure it does), that would be very, very nice too.