I only did VCE Economics and it was one of my favourite subjects, so I can't say much about accounting.
In terms of usefulness for University, I've heard that VCE accounting is pretty useful for ARA, (a compulsory Commerce subject at UniMelb). I haven't done it but my friends who didn't do VCE accounting have said that they needed to teach themselves the VCE accounting content to keep up with the content in ARA.
For VCE economics, the content is useful for introductory microeconomics and introductory macroeconomics, but wouldn't be necessary or be a huge headstart. So in terms of usefulness for university, if you plan to do the Bachelor of Commerce at UniMelb, I would say that VCE accounting is more useful. But both subjects are relatively relevant to the course, as far as VCE subjects are.
Hope this helps!