I have been researching chippers as well. The Walenstein BX62 with hydraulic feed seems like a great machine. Don't know if it functions optimally at 23 HP. Also, I believe that the hydraulic feed requires at least 7 gpm.
I believe that the Pequea CX551 with hydraulic feed can be had for $1,500 less. It may be rated at 5" though, not 6", like the BX62. On the other hand, it has a stepped up flywheel speed, so it may operate effectively with slightly less PTO HP.
There is some info in recent threads about the dedicated 6" Bearcat
chipper with hydraulic feed. It looks to be more machine than either the Wallenstein or Pequea, but also (not surprisingly) more costly (at least $2K more, I believe). Salsco makes a model that is similarly beefy and pricey.
For the 20 acres of mixed conifer and oak forest that I have, the Pequea seems the best compromise between price and capability, especially for a hydraulic feed machine.
Like so many outdoor chores, it really pays to look realistically at what you'll be doing most often. When I look at my woodland, I see that a great deal of the "brush" like material is attached to the smaller trees that I will be clearing and the limbs of the larger trees that I'll be pruning. Hence, the best machine for me is a dedicated
chipper with hydraulic feed. The shredder I can live without, especially if it detracts from the machine's chipping capability or worse yet, means I can't get hydraulic feed for a price that's within my budget. Besides I can always stockpile the occasional bush or sapling and chew them up with my brush hog, leaving mother nature to do the rest. By contrast, if you have lots of true brush and saplings, and need to dispose of them in the cleanest way, a machine with a shredder may be your best fit.