2.) SimObjects folder
Unless you create a different folder and add to in the FSX.cfg .
If you want to install an FS9 model with an auto installer, this may actually be better.
You'd create your "Aircraft" folder within the basic FS-X folder, and then point your installation there.
Of course, you need to add a " SimObject=...." in the FSX.cfg file. Check one of the other "sticky?" threads here.