You'll know which system folder is going to boot becasue the System Folder (MacOS Logo) will be displayed. In 9.1 the new startup disk can allow you to select a particular system folder, but since you're in 9.0.4, it won't work... Take BOTH finders out of their "System Folder" but the 9.1 Finder back in FIRST, and then return the 9.0.4 finder. The 9.1 System folder should now have the MacOS logo.