Newcastle is between the two...?
Well, I live between the two, and next year I'm doing B Laws/Arts at UC and transferring to ANU after a year or so, with my main reason being I'm already accepted for accomodation at UC and I'm too lazy to change it... hehehe...
But there were several reasons I settled on UC as opposed to Usyd, UWS, UNSW, UoW, etc. I had a high enough UAI for all of them, but Canberra is a lot cheaper to live in than any of those cities, plus Canberra has its own economy, thus making it easier to get a job. Which is all kinda important when you're gonna be living like I am next year. Plus Canberra has roads that are easier to drive in, and stuff like that. Canberra's also a little bit closer to home than Syd(Canberra = 2.5 hours, Syd=5.5... ok so a lot closer).
Also, I've been to UC a lot and I think that the campus is well laid out, it has enough facilities(eg car parks, libraries whatever) without there being too many people. Plus everyone seemed really friendly(sorry to sound cliched), and I don't know if you'd get the same thing in UWS. Also UC is really close to Belconnen Mall... hehehe...
Another reason for UC is that being in Canberra there's a good chance you'll be accepted into the public service and stuff upon graduating... I guess it kinda depends on what degree you're doing though.
I spose in Sydney though, there's a much better social life? But I dunno about at UWS, its kinda in the suburbs isn't it? Canberra city's about 10 minutes(from memory... hehe not sure...) from UC, and pretty easy to get to by road/bus. So I don't know if that's a deciding factor or not.
Anyway, ultimately it should come down to which uni you think will do the degree better. Haha hope all this rambling helps!