Can fly or ferry to either one. Not between.
Ferry is always crowded. Ferry, can show up and get ticket, sometimes seats show up, sometimes your ticket is delayed to another one. Go for ticket very early in morning for a later ferry. Weather impacts schedule.
Flying can be more reliable but again limited seats and weather impacts.
Always good to have a second day for flex if first attempt doesn’t do it.
I wrote up a Green Island experience once here. Green Island, Lu Dao (綠島)