I have both. My flail (Mott SHD88 has 108 knife stations, each with two fine cut y knives. Knives cost a few bucks and take 5-10 min per pair to change out. They cut ok when dull but better when new or sharp. I mow slower with the flail, but It's wider than my 5' rotary so prob covers similar acres per hour. Flail gives a much better cut, similar to a garden tractor.
The rotary I use on fire trails, jagging into the edges to keep a field full size, and on unknown ground. I hate to hear the flail knives bouncing off rocks - sounds like a (expensive) waterfall. Rather hear the crash of the hog knives. The rotary is much lower maintenance for me. I can sharpen both rotary knives in the same time as replacing or sharpening 1/108th of the flail. And it cuts saplings better.
Jim