I use to work on black hawks and i had a few mates who did MRH 90 at vary points in their careers.
My advice would be to go for MRH-90 and townsville.
my reasoning being, MRH-90 will at least have a chance to deploy overseas one day. The chances of ARH going overseas are very slim.
If the MRH-90 ever make it to sydney you will have a 2nd posting option. However with Tiger you will be in Darwin for ever and ever.
When you do go away on exercise, with MRH-90 you will be able to fly with it, instead of having to drive everywhere all the time. And from my experiences 5 AVN is a much better work environment than 1AVN.
If you can swing it, I would highly suggest going to Black Hawks in Sydney. They get the better trips, however they do spend a lot of time away from home. You will also learn much more about your trade on Blackhawk. If you are avionics you will actually get to learn to fault find and do wiring repairs, where as on MRH-90 you will just read a computer screen, and wiring looms are changed as a whole with no repairs being carried out. Also civy contractors do a lot of the heavier maintenance on them.
Airframe/engine wise, I believe the black hawk airframe is very similar to the Sikorsky S-92 and S-76 if you are ever interested in going down the civy LAME path one day.