I want to keep my options open, so both
It would be good to pinpoint later down the track what you want to pursue because usually you won't be able to keep both options open: If you do a PhD, then industry is not an option, if you do an MBA, then academia isn't an option. Ofcourse there are exceptions, but that's usually the case.
For an MBA, admission into the top schools isn't too difficult provided you have the work experience, GMAT, and impress in the interview. Of the MBA students that I teach here at MIT Sloan, almost all have 4-6 years of work experience at Big 4's and 750+ GMAT.
PhD admission in the top schools is much more competitive than MBA and don't require any work experience. However, if you are aiming to do a business PhD, I do not recommend doing a BComm since that is directed towards industry jobs in Australia. I highly suggest doing BSci and take as much maths units as possible to be anywhere close to being competitive.