It is a little hard to be sure I'm giving you a correct answer on most power required: Running an 81 hp tractor neither one (hog or flail) drags it down enough to tell really. I do mow on steep enough places a lot to be able to say the flail seems to take a little less "oomph" to do it but that may be because the bush hog weighs about 1500 lbs. They are pretty different to mow with in part because the flail mower has a big steel roller that rolls over the ground right after the flails have done the cutting. The whole weight of the flail mower rests on that roller all the time you are using it without a heck of a lot of weight on the 3pt hitch. You really should find someone with a flail mower and watch it operate some. It is very enlightening.
Full disclosure: I have never sharpened a fail knife. Do sharpen hog blades but only after a LOT of hours. Certainly for a smoother cut one should do that but most of my cutting is pasture fields. I do cut 10 or 12 acres of meadow at least once a year just to keep it from growing out of hand with weeds and other crap. It is what I'd call heavy grass (hay really) which I cut simply because my farm renters are afraid or unwilling to take their big heavy hay equipment on steep slopes.
If you have really GOOD jack stands and can lay on your back, the flails are pretty simple/easy /quick to get on and off. Mine use U-shaped knife holders with a 5/16" pin thru the U - ends and cotter keys thru the end of the pins. I can send pictures if you want. I have changed probably 25 or 30 knives over 3 seasons using it, mainly because of hitting heavy limbs or hidden posts here and there in the pasture. When you knock a knife off the thing vibrates so much being out of balance you know it immediately. There are many brands of flail mowers with all sorts of variations (out on hydraulic arms, offset or not, using heavy solid cast iron chopper knives, using lighter twin knives forming a "Y" and made of 1/8" steel, etc.) They (new mowers) "ain't cheap." I lucked out finding a heavy repairable used one at auction. If your fields to be cut are free of old posts, tree limbs, head sized rocks, etc. then you will love a flail mower. To me they are quite fast and forgiving to operate as well as more maneuverable than the same width hog would be.
Essentially all the highway crews use flail mowers these days. Siddle up to any dealer that sells parts to the state or county roadside people and ask to see some example knives of the various types. I use the heaviest stamped steel knives (as do the state highway crews mostly.) They are fairly cheap and easily replaceable. Mine uses 44 pairs of "Y" style cutter knives in an 88" width of cut. The cast iron chopper types are for heavier brush, etc.
