When i travelled to SE Asia for 3 weeks a few months ago there was very little difference in price between D7 and QR/EK etc.
I decided to fly LHR-BKK and KUL-LHR with QR for the comfort on a long flight and then booked 5 Air Asia flights in between, couldn't fault any of the flights, ontime and allocating seating. Not sure i would manage with them for flights over 2 hours though.