I've only just got my airbrush, so I use that for larger areas and brush paint the rest.
Examples: Cockpit, seats, undercarriage, most ordnance, intakes and exhausts are all brush painted (to get the extra detail). Wings and fuselages and some ordnance (e.g. drop tanks) are all airbrushed. This will probably change as I get more confident with the airbrush, but I'm happy with it this way at the moment.