I'm not a HS coach, but as a fan and parent, I prefer whatever gives my favorite team the best shot. If I'm the underdog, I want a 1-game round. If I'm the better team, then a 3-game series.
I've been in both Georgia (3-game series) and North Carolina (1-game per round, until semis). I probably prefer the 1-game scenario. A lot of these series are mismatches, and it's just a waste of time to play 2 games. The 1-game deal adds suspense and gives the underdog a better shot.
In HS it is all about the calendar, a series just takes too long. Softball/Baseball playoff usually run right up to the end of the year and a series is just not practical most of the time.
I think a final series would be great; sometimes it is just really hard depending on location, plus weather could kill you (sometimes you can wait around and squeeze in a game but not 4 games)