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.
The monoflaps have less leather between rider and horse. As a result, you have a much more sensitive "feel" of the horse. Because there is no inner flap to protect the horse against the buckles under the outer flap, the buckles are lowered below the flap area. No added leather there, either. It's all about getting as close to the horse as you can.
Most monoflaps have stock sizes as well as custom and semi-custom.