I think there's also a fairly difficult decision 5 years from now, should you choose to study medicine. It's quite hard to have a sunk cost of that much time and effort (not to mention debt) and then choose to walk away from a socially desirable career. There's also the 'one more year' issue post graduation, of getting general registration and then fellowship etc. and if you're not careful, your plan to transition into research is stalled substantially. Once you're 30 and have a mortgage and dependents, it's hard to go back to being a student just for a career change.
Whilst many doctors ended up as researchers (someone mentioned this above), they usually ended up there after practising medicine as their primary objective. Studying medicine doesn't make it 'hard' to pursue research, it just adds a lot of time in the manner I described above. It can open doors and give you an excellent clinical perspective but ultimately, that only comes from working as a doctor. If you don't want to commit that much time, it's not unreasonable to not accept your MBBS offer. Nobody can really tell you whether that's best for you; reflect on what you value in a career and what your goals are for life's checkpoints eg. home ownership, city/country of residence, lifestyle.
It's difficult to tell whether you're committed to a particular pathway over the other, so it's hard to offer much advice but mapping out what you want to achieve/obtain is probably a necessary step.
As an aside, this situation is a product of asking 17 year olds to commit years of their life to careers they know nothing about. Melbourne made some mistakes when they transitioned to focusing on graduate study, but I firmly believe it's a substantially better educational model.