I use a stihl FS240 with harness. The one problem with brush cutters is that moving the cut brush is difficult. You have to unclip the cutter from your harness. If you have thick brush where you often need to pull the stuff you just cut out of the way to get to the stuff behind it, the cutter and harness is a lot less useful than a chainsaw that you can put down and pick up more easily.
It's also not good for woody stuff that's got much of a diameter to it. Anything over about 3/4-1" and I'd rather use a chainsaw.
The Garlon 4 in diesel works well but you have to apply it when the plants are growing (it works by disrupting growth).