Quick answer: A sign spinner helps when movement can grab attention at one spot. A still sign works when people already have time to notice and read it. The spot decides, not the artwork.
Getting noticed is not the same as being understood
Movement gets the first look. But too many words still ruin it. A still sign can be easier to read if people are waiting or moving slowly.
The spot changes the answer
Think about which way people approach, what blocks the view, how long they'll be there, and what's allowed. A plan that looks good on paper can fail on the real corner.
A person needs a plan too
A sign spinner is a real person. That means a schedule, breaks, weather plans, and training. A still sign needs a different kind of setup and check-ins.
Pick the simplest fix
Don't pay for movement if a clear, well-placed still sign solves it. Don't rely on a still sign if it will get lost on a busy street.