ANU seems to have a fairly good rep for Econ, maybe not so much for Commerce. I don't study in either of those areas so I can't tell you more than that.
If you're in Sydney, ANU would mean moving out of home. This has its advantages and disadvantages: the main advantages being that you get more independence and you can experience all the fun of college, and the main disadvantages being the cost (and possibly having to cook if you don't live catered). I live in college at ANU and I haven't regretted it. Canberra isn't that bad, although I say this from the perspective of someone who's lived all his life in regional areas. It may not have all the nightlife and culture of the big cities but if you're not into that it really isn't a bad city. We have light traffic, clean air, lots of parks... it's a nice place.
I strongly recommend touring the campuses and any accommodation you're looking into, if you can. It'll allow you to get more of a feel for the place and see which one you prefer. If you want to know anything more about ANU feel free to ask me.
