Flail mowers are tops for addressing many of the points for using either a rotary cutter or rear mower but it is a more expensive proposition (and much more expensive if you want to get a good one).
It really boils down to two key factors:
1. What do you want to accomplish?
2. How much money do you want/have to spend?
If you are filthy rich, get one of each... /forums/images/graemlins/grin.gif... sorry, couldn't resist.
If you are just looking to clear brush and keep the grass wacked down, then an rotary cutter (brush cutter) is the way to go ... and it's the least expensive solution.
If you have a really nice lawn that is mature in terms of it's landscape being free from debree, then you should consider the RFM. While this will result in a very nice look, I am not an advocate of this solution. I would consider the reel mower pull behind as you can get them such that they cut a much broader surface at once (like for schools) and that makes for a nicer cut and quicker work.
However, if you have something like an orchard and you want to keep the grass down while knocking down the occasional small bush and you want it to look very good (say you have pick your own) after you're done, you may want to consider the flail mower (my favorite .... grew up on a farm/orchard/vineyard with one but can't justify one at this point).
I happen to be quite pleased with my 6' rotary cutter because it is an excellent all purpose cutting attachment. I manage about 10 acres with it annually (that's a pretty big chunk) and it does a very nice job. I just purchased a new one this year (my old one fell apart).... someone made a comment about keeping rotary cutter blades sharp but I've come to learn that it's best to purchase new ones (personally, never had to do either and I have knocked down a lot of brush).