You will need to log in with each user and specify this in Applications -> System Tools -> System Settings -> System Info -> Graphics --> Forced Fallback Mode. Fallback Mode is old GNOME interface and the other Mode is GNOME Shell (standard) mode.
For other desktops:
You do the selection using your login manager. It should have an option to select which session you want to log in, and if you change from default, it should ask if you want to permanently use the new session. Session in this case refers to any Desktop (GNOME, KDE, XFCE, ...).