Heya,
My best advice would to be visit all the university open days that you are interested in! I visited UMELB, ANU, USYD and UNSW whilst I was in Year 12. I have many friends at ANU and at Monash. So you can get a feel for it yourself. Also where are you from originally? For all the major cities, I had to move so thinking where you live is really important. You can tour the colleges at ANU (ANU also has a first year accommodation guarantee for first years, and a wide range of colleges from self catered to fully catered) and there are also residence at Monash. (I have friends at residence halls at both ANU and Monash if you have Qs)
Are there any Monash or ANU scholarships you may be particular elligible for such as the ANU Regional Scholarship? That's something to look at to when you visit at open days if you are split between the two (i.e love the course structure and the cities equally).
Canberra and Melbourne as cities have massively different feels to them. (How you perceive them probably depends on where you grew up/went to school
) Canberra is a much quieter city, but has a lovely friendly vibe to it and is the hub of the government. If you lived in Canberra, you'd probably want a car as it is quite spread out, though biking is a big thing there to get around. Cold weather, but much drier than Melb.
Melbourne is much busier, and really depends what part of the city you are at. There's a real artsy section at Southbank with the arts markets and that, where some of outer suburbs really differ to what you find in the city. Monash is kinda out in the boons in Clayton. Fun busting city though, temperamental weather with a random shower can be really hot and really cold.
So my advice is to visit both yourself and see, and also look at which offers you the best Arts/Science course structure (look at the units you can take and what you can major in, and how long the degree is, though iirc it's 4 years at both Monash and ANU). Look at exchange partners if that's something you are really interested in, ANU has amazing language programs and labs due to the being the hub of Asian studies and international relationsy stuff as it's all governmental. My sister did Indonesian in her Masters there. Monash and ANU both have great specialities in science, like depending what you are looking at, ANU has really great physics (Hans Bachor is the best prof ever there lol).
Hope this post is somewhat helpful