Answer by Micheal Chang:
The air space between the two won't actually matter much at all.
The transmission loss will be primarily provided by the double pane window assuming proper installation and weather sealing. The rolling shutters will contribute to the total attenuate at higher frequencies, but virtually nothing at lower frequencies due to its low mass and absence of tight seals.
Given the min/max distance constraints on such a window installation, I would just go with whatever is the most convenient from an installation and aesthetic viewpoint.