To me it is not as simple as choosing QF or not. Each flight is different and I will choose the airline fly with based on several criteria. Things like company policy, cost, departure time, travel time, number of connections, level of service, type of aircraft, seating, luggage allowance, FF points, FF status benefits and lounge access. I don't have a lot of loyalty to any airline and will happily switch to which ever one suits me best at the time. But having said that I do give Qantas a slight favoritism.
For domestic and trans-tasman most of the time for me Qantas is coming out top in that equation. But Virgin is closing the gap.
For other international destinations Qantas usually doesn't rate well. Primarily because i'll need one or two more connections to go with Qantas than alternative airlines. Or even that Qantas don't have any overnight flights from the east coast to SIN or HKG, or conversely no day time flights coming back the other way.