aaahhhn!
since it is a really cheap american building, i choose the 3mm one, even if i prefere the 5mm one, it is 2 times expensive!
the one i took is a poor quality one, with a bad foam inside. just good for the walls.
i have a small piece of 5mm with a good foam inside, and i'm planning to use that for same parts, like shown in the MAKE IT REAL article, from a FRENCH GENIUS (look on the "links" widget on my site! =) )
i'm also planning to use MDF for some concrete parts, I'll make a try in this days!