Well personally since I am a slow-scoring batsman I would choose to chase the 239.
But seriously, if I was Australia I reckon the odds would be quite similar for both. Yes, I can see that most peoples gut feeling would be that the 239 is easier, but if you look at the stats on thousands of games (which D&L have done) you would find that the chance of winning either scenario would be very similar. These days 239 is considered a fairly low total and in lower total games the number of overs become less relevant as you basically need to bowl the team batting second out to win. So the runs don't go down as much as you might think.