I've used both silicon and contact cement. The contact cement is a much better bond hence why it's permanent. If you want to get the protector as close as possible to the rail the silicon will not stick well enough around the curve of the board.
I have removed a protector stuck with silicon and is still leaves quite a mess. You have to scrape the silicon off with something sharp which risks the paint being scratched.
I think your better off using contact cement and doing a good job of it. The Starboard protector fits in nicely with the design of the board, so there's no reason to take it off.
Just mark the area with a pencil. Apply contact cement to board a and protector and let it touch dry. Then carefully align one part of the protector to the pencil markings on the board and progressively stick down the rest. Be aware, as the name suggests contact cement, bonds immediately on contact. So you need to get your alignment right.