There is nothing wrong with Gardtec gear IMO. The 500 and 800 series are excellent panels and are far easier for the customer to use. They are just as well built as the Galaxy. The LCD keypads are much less "commercial" than the Galaxy as well. Gardtec would not be my choice for anything other than a domestic/small commercial system, but only because the Galaxy is a more flexible panel engineering-wise. At the end of the day, the end user (yourself!) just wants a reliable, easy to use system that looks nice and does its job. Both systems offer all of these qualities. You should choose your installer based on your perception of them as a company (ie polite, professional, informative). Leave them to worry about the equipment. It is after all in their interest to fit quality equipment (I am assuming that you have picked approved companies on that last point)
Scott (experienced Galaxy and Gardtec installer)