If you select Tower you are signing up to be posted to any of the 28 tower locations currently staffed by ASA, you are also agreeing to be transfered throughout your career based on operational requirements. Tower is by far the best option, however if you're not prepared to live outside Brisbane or Melbourne then go enroute.
If you can't live in WA then i would suggest not going Tower you have at least a 25% chance of getting a spot there (all tower groups currently have at least 1 person going to WA 'slated')
I wouldn't bank of the "bid for 5 positions" crap. The last tower group were given 8 (out of 28) tower positions and asked to give 3 preferences, so your options are already quite limited. You have almost zero hope of getting any brisbane based position given the current waiting list of experienced controllers for Brisbane based positions.
At the end of the day, if your given an offer for a tower course and turn it down, you will be kicking yourself for the 12-18 months your at the college, if not the rest of your career, just don't go in expecting you'll get a certain tower, not only will you be disappointed you'll piss off the rest of your course mates and instructors