Hi Paul !
The short version is: Tamiya for airbrush and Vallejo for paintbrush. Both paint brands are excellent when applied that way, they are rather rubbish if you do it the other way around though.
Vallejo does however also make "Air" versions of many of their colours, and these are sort of intermediate not great in any of the two scenarios but will be OK in lack for better alternatives, plus they have a much wider colour-range than Tamiya.
If you start out with Vallejo model color, and supplement with Tamiya once you get your Airbrush, you'll be fine. Even after getting an airbrush you will still need plenty of good quality brush-paints for detailing.