A couple of things to consider:
1. The likelihood of an A330 being subbed to a B737 due to light loads (not sure what the onward pattern for the bird is ex-QF430, but would have thought this possible for weekends
2. Possible delays. I avoid the 6:30pm SYD-MEL during the week because the few times I've taken it, it's been delayed inbound or not turned-around promptly or both. I now book the 6:45pm B737 as this seems to be more consistent and (IME) earlier into MEL.
Regards,
BD