Depends on the retracting speed you would like. If you're doing something vertical the mechanism with a single acting should drop fairly quickly on it's own via gravity depending on the weight of your prop and the amount of air released if you are going to control that at all.
Double acting you can control the extension and retraction speed of the cylinder making for a swift motion both ways if that's what you prefer.
I have props with single acting and props with double acting, just depends on what you're going for.
What are the planning on doing with the scissor mechanism?