I try to keep myself to 1:500 scale and to planes I've flown on. I've made the occasional exception for things like a 1:600 Thai A340-600 because I haven't been able to find it in 1:500 at a price that was $239 or so.

One of the things I looked forward to the most about my first Lufthansa A380 flight a few years back was ordering the Herpa Wings model from the duty-free cart in-flight.