I ride in an Antares mono-flap and I really love it. Of course, it was custom fit to the horse I ride so any other saddle would be sub par for him, but that's beside the point.
The important thing is to find a saddle that fits you and your horse. The mono-flap is less leather to clean (always a plus, imho) but also requires a certain type of girth (more $$ to shell out). And, as far as I know, you can usually only get mono-flaps as a custom, but I could be incredibly wrong about that.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."