Personally I find that the bulb shape gets in the corners of my lather bowl better, and fans work great for soaps and face lathering.
One advantage of a bulb shape is that you can have a brush with a larger loft with out it being overly floppy. This can be very positive if you like to bowl lather with creams.
Why choose one over the other? No need to. Getoneseveral of each.
-Andy