I've rented both in the past, and the big one is amazing! Drag the tree to the chipper with the tractor, lift the end into the hydraulic wheels and in 30 seconds a 30 ft tree is gone, branches an all. Of course, I was chipping freshly felled stuff, so it was still plenty green and soft. I rented the Vermeer from a reputable place (Hertz equipment rental with the Vermeer dealer next door) and everything worked well - had a Perkins diesel engine on it. Picket it up on a Friday afternoon, used it all weekend and brought it back Monday AM, got charged for one day.
The 6" was fine for just doing limbs or saplings, even pretty big limbs. But, it took a bit for the engine to get the flywheel up to speed on startup. Once there it did fine.....
I did manage to clog the chute on the big one once, I think I did a bunch of cedar or osage orange. Just tipped the chute over (with it shut down of course) and shoved it out with a branch, then back to work. 2 guys cleared a LOT of trees off my property in one day.....used the tractor bucket to deposit the chips around the bases of the remaining trees as mulch.