Domestic I go with whoever my employer utilises at the time or whoever has the best deal.
VB are now fully competetive with QF so barring a major difference (eg prop v jet) the time spent in the air is pretty minor. Not so long ago it was always a stipulation to fly QF.
Internationally I book my schedule around the 380 because it is just a better experience. QF usually gets me so I can upgrade on points but I've started using Singapore and Emirates more recently because they are just happier, or perhaps that should be less miserable.
If you missed the meaning of that I choose to pay for a business class ticket with carriers other than Qantas where I would only have to pay for an economy flight and then upgrade it.
That's pretty telling as a judgement of what is delivered.