Both are soft, diffused light modifiers, but their shape affects how the light looks and how easy it is to control.

An octabox tends to produce rounder, more natural-looking catchlights and reflections, which is why many portrait and fashion photographers like them. On reflective subjects, the curved shape can look less obvious than the straight edges of a rectangular box. Octaboxes can also feel a bit more focused/directive than a typical rectangular softbox, so they may put slightly more light on the subject.

Rectangular softboxes are often easier to control precisely because of their straight edges. They’re useful when you want to mask, flag, or feather the light in a more technical way, such as product photography or lighting parallel to flat surfaces. They’re also often simpler to set up and sometimes less expensive.

A practical rule of thumb: use larger octaboxes when you want a natural, broad portrait light with unobtrusive reflections; use rectangular softboxes when you want cleaner edge control, more geometric reflections, or lighting that matches the lines of your subject. Either can work well—the best choice depends on the look and control you need.