Stuggle may be too strong a word but there is a definate difference in fuel per pax seat km between the ATR and Q400 which gives Virgin some room to move on price. It depends on whether pax notice or care that the flight takes a bit longer. In the case of Emerald/Brisbane I believe it is about 20 min difference. Not to be sneezed at but it depends how much you are prepared to wait to save some money.