Both add the animated captions that short-form demands. Captions is the broader app: teleprompter, eye contact correction, dubbing, editing on your phone.
Submagic is narrower and better at the one job — captions, B-roll and zooms that look designed rather than generated — and it costs less. Pick Captions if you shoot and edit on mobile; pick Submagic if captions are the only reason you opened the tool.