Now there's a big question with many answers. I would suppose it is all down to what you are most comfortable using. Some modellers purely use a variety of paint brushes, others will use nothing but an airbrush and then there is those who use a mixture of both. The more you paint using different mediums then the more your experience grows until you know yourself exactly what works best on what materials and on different surfaces.... Best thing to do is practice using both.

Regards

Alan