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Driving 4 hours across Los Angeles and up Interstate 15, east into the sun through a lot of featureless desert (barring garden spots like El Monte or Barstow), after an overnight transpac from Australia, when you're accustomed to driving on the opposite side of the road, may not necessarily be the best option.

On the other hand, there won't be much traffic and the car will probably be cheaper.

From LAX, Southwest (WN) has the greatest frequency of flights and the most liberal flight rules about flight changes/refunds. On the other hand, you won't be able to interline your bags all the way to LAS as you would with United (UA) or US Airways (US). Depending on the fare, there's a possibility you could accrue NZ Airpoints with UA or US, if that's of any interest. WN, UA, and US all fly out of different terminals from QF, so you'll need to switch terminals one way or another.