I've had both. Agree with what TerryR said but, in addition, you cannot beat the torque of diesel power which you typically do not get in a separate unit. Think I've only ever stalled my tractor mounted unit unit maybe one time; whereas, I was forever stalling the old Troybilt
You only need 18.5 hp to run a 3 1/2"
chipper. That's what I had when I bought my first PTO unit, a Mackissic TPH-122 (TPH = three point hitch). Now have 24.5 hp because JD refuses to make a smaller tractor running the same Mac. In fact, I recently completely replaced all the hammers and spacers after about 12 years.
I also use mine for shredding leaves. Think a hammer unit is better for this vs. an all knife, mainly
chipper, unit. I pick up over a 100 bags of leaves that town homeowners just throw away (and our local business turns them into mulch, compost or soil that he sells back).
I highly recommend the Mac TPH-122 if a 3 1/2" and smaller unit would meet your needs. It is so easy to work on. Before, I had a Troybilt Tomahawk, which is an absolute beast to work on. Why? The spacers between its hammers get beat and beat up and misshapen such that they "weld" themselves to the shafts. You can then spend hours hammering those shafts back out. (Maybe, if you had a 2nd person applying a torch to those spacers, it might get easier.) The hammers on both the Tomahawk and the Mac are the same: 4 corners and 2 shaft holes. You can rotate them 3 times when each leading corner gets rounded off.
Chipper blade changeout on the Mac is easier, too, once you know how, because you don't have to reach inside to get a backer tool onto the head of the bolts holding the
chipper blade. On the Mac, they're screws.
ALWAYS use a face shield and long gloves. I once had a hand pruner thrown back out at me from a bunch of leaves I lifted with 2 hands and let dribble into the Mac. Didn't catch the hand pruners in there until they caught ME coming back out.
Ralph