Keep in mind that what you see now and what you expect to see in the future are likely going to be different perspectives. I'm currently faced with having both a lot of brush to clear AND having a lot of grass to cut/maintain. I've found bush hogs to do an amazingly good job at cutting grass to a near-lawn cut. Interestingly, while sharp blades DO tend to make nicer cuts, dull ones (NOT bent!) will amaze you as to how well they actually cut. The phrase "speed kills" is quite apt here in that the speed at which the blade tips travel is insane (14k to 16k feet per minute).
I've run a five-foot bush hog on my
B7800 for over 7 years (hundreds of hours with a rotary cutter, perhaps 500 or so). The
B7800 is a 30hp tractor (about 23hp at the PTO). In heavier stuff you just have to slow down (doesn't matter if you're running a small or large tractor): early on I was cutting grass that, some of which, was upwards of 9' tall! The diameter of material that one can cut is mostly based on a constant cutting. Hardly does one look to chew through LOTS of large diameter material. It really comes down to speed. One could cut larger diameter material given enough time. Of course, in order to cut something you have to be able to roll OVER it first! This is likely the more limiting factor: if you try to push the speed you'll likely start to push the implement and tractor to the point of breaking something. One does NOT want sharp blades when cutting brush, for not only the reason noted by japody (above), but because shattering stalks/stems is more effective in that it does more damage/stressing to that which you are trying to eradicate.
As I started saying, I've got a lot of brush AND a lot of grass. I'm kind of straddling the fence with implement needs. I'm on the verge of getting a flail mower, for, at least, the
B7800 (so the wife can do some mowing- much easier and safer to mow with). A flail mower can produce a very nice cut; and, it doesn't, and this is probably the biggest factor, leave clumps of cut up material (as will a rotary cutter- even a regular lawn mower will produce some windrowing). Fails can also handle brush, though usually lighter stuff. They're nearly 3x the cost of a rotary AND they require more maintenance. But, it's all in what tool works best. My NX with 6' rotary cutter will, for now, be the main brush clearing setup.
