Thanks for the help and comments. Looking at the uncertainties of baggage interlining in this case, and chance of a delay leading to a missed connection, I have routed MEL-LAX-IAH. Indeed, the difference is not that great. I might well be over the QF baggage limits, and...
Remember also, that Australians don't have Global Entry, even though we let USA people use SmartGate - isn't life unfair?