depending on what grillo you have. open differential? Does it have steering brakes, I would assume brakes are better than un-engaging the drive wheels. Do you have enough HP to run a flail the same size that you have. or do you want a bigger unit? Is your dedicated brush mower a hydro-static belt drive? Do you have alot of back and forth cutting where a hydrostatic would be helpful?
Lastly, the Grillo PTO doesn't work in reverse, do you cut in reverse?
Best of luck.