Deep seat saddles are for riders who like to feel a little more secure in the saddle or like to sit deep most of the time instead of two point, as its a harder to two point in a deep seated saddle, which explains why a dressage rider might prefer it.
Flat saddles are for the rider whose butt is out of the saddle most of the time or don't like to feel quite as "deep" in their saddle, and half deep or med deep saddles are somewhere between the two.
Most close contact saddles have square cantles. I'm not sure exactly why but that is the fashion in the h/j ring. Round cantles are more likely to be found on a/p saddles or dressage saddles.