On a practical note, we have to let some hedges go 5 years because of the Environmental Protection Scheme (REP's) so it can be quite a handful to get them back into shape. The only problem with the saw is the cleaning up after. At least with the flail your finished. I see there is a rotor blade available (all the world like a rotary lawn mower blade only heavier) instead of the saw for light work, but I'm not sure how good they are? If your going for a flail type make sure it has the heavier T bushwhacker flails. They should be able to handle 3 years growth if the direction of rotation is down into the hedge.
Have you good quality hedges or are they like ours a mixed bag? I guess the forward throw on the arm would make a difference at the end of a long day. But in my opinion stick with the big two manufacturers and you cant go too far wrong regardless of which model you go for. And a good gear range in the tractor and a wet clutch for longer life.
I hadn't really thought of Cheryl and the Tattoo project at the same time, but now you come to mention it................