I am told that Scandinavian countries and Germany have banned chemical gases. this is part of the 'KYOTO' Protocol. Big debate at the moment on whether this will effect FM200, FE13, NAFS3 etc. One thing you would have to have with a gas system is a low level extract system to purge the area after discharge, this could be a problem with a below ground installation. You cannot just open the door and let it out! Also, you cannot discharge it at street level, it has to go above street level. I would stick with the fog system. No purging, no environmental impacts now or in the future, no room integrity issues, etc. As you probably know there are several good systems on the market now. The ones I know that have been tested on switch gear and have the reports available are Fogtec & Hi-Fog (Marioff).
Hope this helps.