Q400's are used to HTI which have an excellent on-time record (better than the Q300s used on some other routes). VA is more likely to have trouble with a B738 than Q400 so I would stick with QF. Worst case you may be rerouted via SYD which has a later flight to HTI (but schedule varies a lot seasonally). It is Lord Howe Island that is problematic for flight cancellations due to the nature of the airport and weather, HTI is pretty easy landing.