I can't speak to the air pin box, but I have a Trailer Saver air ride fifth wheel on my GMC 6500 mdt and couldn't be happier with it. I tow a 23000# 40' trailer with 5000# pin weight and it rides very nice. My truck has leaf springs and I am convinced without the hitch it would beat the trailer to death. You can push up on the front of the trailer with your hands and easily bounce it up and down about 3", so I figure every bump is getting absorbed by the hitch. One plus to a hitch vs. a pin box is that I have it hooked up to a air switch in the cab (with air tank and compressor) and when I unhook all I have to do is crank the landing gear until it touches the ground, then dump the air out of the hitch and pull out. Same when hooking up, back under the trailer with the air out, latch it, and air it up and the landing gear is already free, just crank it the rest of the way up.
I also didn't want to bite the bullet on price, but I wasn't in a hurry and watched ebay on a daily basis. I missed a couple of them but finally got a used one in like new condition on ebay for $1500 which I figure is a bargain as they are $3000 new, and particularly in my case as I was looking at $1000 for a new Reese mdt 30000# hitch anyway.
If your truck has leaf spring suspension, you absolutely need some sort of air cushion between the truck and the trailer. Fifth wheel campers just aren't built with that kind of truck in mind, and there are some horror stories out there of bent frames and pin boxes on the trailer from towing with a hdt. If you have air ride on the truck, not so bad, but you will have to be able to adjust the air pressure appropriately for the light load of a camper or they are still rough. In the campground we are in right now there are 2 hdt's plus my mdt and all 3 have the trailer saver, that should tell you something.
check the hdt rally forum, there are some great tech articles on there as well as the escpapees forum.