Will you be mowing close to people or buildings? If not, and with brush as your material, a RC would work fine and probably cost less. For a flail your best bet would be hammers (not scoop knives, which are sometimes mis-advertised as hammers) for durability and mulching of brush. A quality brush cutting flail with true hammers is probably going to run US$5000+. There are several good lower cost flails for grass and pasture mowing, but not setup for heavy brush cutting.
I love flail mowers for grass and mixed weeds, and safety, but for brush in areas without stuff to damage, RCs have their place.
If you go RC you'll want either a 6' or a dual spindle 8' (two x 4' in one housing), because the 7' models are uncommon and huge. If you go flail you can find something within a few inches of 7'.