Photo gear and marketing hype go hand in hand. But there are just a couple of factors that determine which shape of modifier I’ll choose for a shoot. Number one is the effect of the catchlight in my subject’s eyes. I prefer a round catchlight—maybe just a subconscious preference for a light source shaped like the sun. If I were simulating an open doorway or the light from a window, a softbox would be my choice instead.
There are a bunch of other considerations when choosing a modifier that are partly determined by the shape of the modifier but also by the material and construction of the modifier itself. I may decide I want directional lighting, with harsh, defined areas of light and shadow. Or, I may opt for a soft, diffuse, enveloping quality of light that wraps around my subject. This has less to do with the shape of the modifier than by the size of the modifier relative to the subject (larger=softer light).
For me, the shape only becomes a deciding factor when I want to control the light precisely and thus I might use a square softbox rather than an umbrella because I want the light to have a more defined fall-off or edge.
You can find many descriptions elsewhere of the effects of different light modifiers, but I like this resource from light manufacturer Paul C. Buff that clearly shows the effects of various different light mods.