This no longer works the way it used to back in older versions when System Settings was System Preferences, i.e.
set the current pane to pane id "com.apple.preferences.softwareupdate"
I had a little applescript saved as an app that would open Preferences to Software Update, and open the App Store and send it the keystroke shortcut for its Updates tab (since App Store is apparently not properly scriptable). Handy to quickly check both, but doesn't work anymore :-(