I've never owned a flail, but I have used one a few times. Right now, where I'm at with our property, a flail would be my choice. (snip)
I have pretty much cleared the parts I am going to clear and am in maintenance mode now. That's where the flail comes in. All I will need to cut is tall grass, weeds, and light, tender sprouts. I used the DR walk behind brush mower last summer to maintain, but it takes a long time at 26 inch cut, and beats me up a little. Since then, I have cleared maybe 1/8 acre more.