Another vote for the southwest. Try to spend some time in the enchanted Forrest in boranup - just out of Margaret river. Or heading down to the big trees in pemberton or Walpole.
But you will prob nee three days min for that job of a trip and still feel like you are rushing.
Also the NW and the SW are like chalk and cheese - you could even argue they are different countries with how different the climate and geography is! Would be good to see both but the distances are vast