Unless you are a sucker for elaborate designs, the circular shape is the way to go. It keeps the center of mass balanced where your grip is, which makes it the most responsive to move in any direction to intercept a strike, particularly if you rotate your wrist at all when using it, as well as being the most stable when it gets struck. If you decide to go with something more stylized, my recommendation would be to give it rotational symmetry, rather than a teardrop or kite shape.