For tall grass and some blackberries it sounds like a rotary mower, faster and less HP requirement giving you a bigger swath. They're more common and cheaper.
The flail would shine if you wanted to mulch brush, it'll chop it up finer. If you said you wanted to do more blackberries than grass, then I'd suggest a flail.
edit - what Bucks2Ducks said, if you got those blackberries dealt with you could keep it nicer with a finish mower.