So what you are saying is you have 2 machines and either one will use the attachment ? ..... I was currious ...a have used both styles of mulchers or chippers or mowers ??? I guess what I am saying is what ever you are using it for is what we will call it today ...LOL .... I own a rotary disc with flailing knives (4) on the outer edge of the discs which are 4' in diameter +2' for knives (6' dia tot) ..... the knives are about 20# ea. the whole disc assembley is about 1100 #'s ... now when this thing starts to spin, virtually nothing will stop it except over feeding it (to fast) it takes alot of hyd hp to keep it going. I happen to use it on both my A-300 and on my Cat 312 excavator ...... this thing I have called "The Destroyer" has the ability to take down 8" dia. trees .... (I also do this for a living) ...... when you hit Rocks with a fixed tooth or flail toothed drum type something has to give ? ....usually the teeth shatter or break, and not just one at a time, the one job I was on a year ago I used one of these $42,000.00 units on my Cat 320clu (65gpm) and went thru $3,000.00 worth of teeth in 10 days due to the rocks camo'd in the growth ...... I just finished a contract, with the rotary, and did 35 acres on an Airport of 35 yr old growth of willow, fir, bug kill pine, aspen, fire weed grass crap and old fencing too, and never wore out the knives but having said that tho, we did have alot of growing pains along the way, mainly I now have my knives custom made, (not the OEM AR-400) am now using a grade of Swedish HARDOX steel. ...... these rotary units are about half to a third of the cost to buy and a way lot cheaper to maintain ..... its just my opinion about these attachments that I back with experience .... if I am wrong about this simply prove it to me, Show me where the BEAR Craps in the Bush .....Good Luck and hope this helps ...... GK