For most real-world use, the diffuser’s shape matters far less than its size and distance from the subject. A diffuser softens light by making the apparent light source larger, so those factors have the biggest effect on softness and spread.

In practice, square vs. octagonal mainly changes the shape of specular reflections, especially catchlights in eyes or shiny surfaces:

• octagonal: rounder, often seen as more natural-looking
• square: more like a window reflection, which can be desirable stylistically

Aside from those reflections, you usually won’t see a major difference in the light pattern between a square and an octagon, especially with small on-flash diffusers. Their corners may not even be lit evenly, reducing any theoretical shape difference further.

So if you’re deciding between the two, prioritize:

1. overall size
2. how well it mounts and stays in place
3. how practical it is to use

Also note that small on-flash diffusers generally have limited ability to create truly soft light for portraits. They can be more useful for close-up or small-object work, where the light source is relatively larger compared with the subject.