Start with the reason
Hiroshima and Miyajima need pacing. Hiroshima carries historical weight; Miyajima adds nature, shrine, water and slower rhythm.
When one night works
One night works if Hiroshima is the main purpose and Miyajima is a bonus. Arrive early and keep the plan focused.
When two nights are better
Two nights are better when Miyajima should have real quality, or when family, luggage and hotel changes need margin.
Miyajima is not only a photo
Plan the island as a place, not only as the floating torii image. Weather and tide matter, but they should not decide the whole value.
Practical choice
Use Hiroshima as the simplest base unless the island stay is a clear priority.