I really wouldn't bother with the airport hotel.

It's about 20 minutes to downtown in a cab. Pick a hotel near where you want to be that evening, spend a couple of hours wandering around down there then back to the airport in the morning. The only time you'll save is immigration which is usually very quick anyway. Security is cleared at the gate so you have this regardless of where you stay.

Note that the train doesn't run that late so if you're heading back much after 11pm you'll be in a cab (or a bus) anyway.

Also if for whatever reason you can't get your boarding pass until the next morning (as referenced by allset2travel) you're going to be stuffed as you won't be able to get back airside.