I've used many flails over the years. Seppi, Rears, TriMax, Befco, Alamo and a few that I've probably forgotten. I prefer the Befco for several reasons. The housing is a little taller than some which means that there is more room for tall grass to stand up and be cut. Flails give their best cut close to the ground, by the way. And most importantly, you can specify rotation when ordering.
The direction of rotation is very important. The best way to describe flail rotation is whether it spins the same as the tractor tires, or opposite. Opposite rotation cuts and re-cuts the grass providing a very through mulching, useful on a lawn. The two drawbacks are that it can require much more horsepower in tall grass, and it may not cut tall stems like raspberry canes, etc. The tractor tires run over the grass, laying it down in a forward direction. With opposite rotation, the cutters can deflect tall material, instead of cutting it.
I prefer the cutter to rotate the same as the tractor tires. When the tractor tires lay the grass down, the rotating cutter picks it back up, cuts it clean, and spits it out the back, where it lays it down nicely. I use the flail on unused hay fields where the grass stands 2-3' tall. Using a flail that rotated opposite the tires, 100 HP and low gear still wasn't enough. With rotation the same as the tires, a 50 HP tractor did the job in third gear without bogging down.
Befco flails can be ordered with a choice of rotation, and can be changed later on (by a good mechanic). That's why I prefer them.
If you are doing tall grass, a flail works best rotating the same as the tractor tires. It cuts the grass nicely, because it picks up and cuts what the tires lay down.