The two basic shapes of a shaving brush knot are the bulb and the fan, there are reasons for this and they both behave differently.
A fan shaped knot puts more bristles in contact with your face, so for a given size it will massage and face lather more effectively, also it will pick up more of a potted cream or a soap when you first charge your brush. Everything else being equal a fan needs to be made with a shorter loft if it is not to be too floppy.
A bulb shaped knot has a centre that is supported by the shorter hairs around it, this means that it will have more backbone or can have a longer loft for the same backbone. It will bowl lather more effectively. Obviously the supported centre will have a smaller contact patch on the face. The bulb can split more easily into two sections with a gap between them when you are picking up a hard soap or when you are massaging your face firmly.
When I look at my brushes I see very few that are pure fans or pure bulbs. They mostly seem to be a mixture of the two elements as the designer has tried to give the brush a certain set of characteristics. This may be a compromise or it may give you the best of both worlds, the only way to find out is to use them to see what suits you.