I have a Vermeer PTO powered 3 pt chipper with hydraulic feed. It has a 6" capacity. I have never seen a more heavy dutyn3 pt chipper in this size range, the chipper weighs in at 1400lbs, which is about double what some of them are. Vermeer no longer makes PTO chippers however, but if you ever came across a used one in good shape I can highly recommend them.
As MM said, hydraulic feed really is nice when chipping, even on the smaller branches. Of course, it adds considerable cost to the chipper unfortunately, especially the fully self contained hydraulic models (pump driven off PTO so no tractor hydraulics needed).
I did a lot of research on chippers before I purchased mine. I wanted to purchase a used chipper to save money, as I couldn't justify the high cost of a new one. I decided that if I couldn't find a chipper (had to be hydro feed) within a year of when I
started searching, then I would pony up and buy new. I settled on the Wallenstein BX62R as the one I would purchase. Wallenstein has many many happy chipper customers and the thing that convinced me to buy that one if I had to go new, was the rectangular feed chute. It has a 6" capacity but the feed chute is actually 6X12, so I could fit slabs in there from my portable sawmill and not worry about the width. I've also read that it is helpful with limbs that have a lots of branches coming
off them as it is easier to feed, however I don't have any personnel experience with this.
A lot depends on tractor PTO hp, capacity needed, and of course budget being the most important. These chippers are expensive, and sometimes it makes a lot more sense to just rent or hire it out. I myself really like having the chipper around and will probably never sell it, I know it will outlast me with proper maintenance.