Several threads going on about this.
The key word to me is that the tractor is a hydro. That means you can ease off the pedal to keep the rpms up when you get in tougher stuff. Engine rpms stay up as does the blade rpms. No bogging down. I would definitely go with the 6' but would not spend the money on a heavy duty one. With 35-hp a light or medium duty mower will be all you need. And with a six footer you will be cutting a wider path at a slower speed and will have a more comfortable ride on your tractor. We have to mow one patch of about 20 acres CRP land that has to be disked every other year and it gets rougher each year. It will shake your teeth out. A wider mower and slower speeds is the way to go.
I have no experience with flail mowers. I don't know how well they hold up or how much debris, limbs, saplings, roots, etc. you will be mowing up. I do know how a well taken care of rotary mower will last. One of the mowers we use yearly is a 6' Ford that is 40 years old. It has been welded, chained, bolted, and screwed back together so many times we have lost count. Finally filled the gearbox with two or three tubes of grease to stop a leaking bottom seal. Leak was caused by a nephew mowing up a couple hundred feet of electric fence wire which wrapped around the shaft............ OK, go back and strike out the words "well taken care of". The mower has been thru he11 and still works.
RSKY