Science is broader and has more options. You can do majors in chemistry/maths/biology/psych/etc., whilst Biomed really does limit you to the health sciences. Biomedicine is a course streamlined to develop students for careers like medicine, whilst Science leaves you to set your own course (more so).
If you get >99 you're guaranteed a full fee spot in the MD if you maintain an H2A average and pass the interview. If you get >99.9, it's a CSP spot.
The GAMSAT is brutal. It's an exam that runs for an entire day and a competitive score is probably about top 10% or so. If you put in the study over the summer holidays, achieving at that level is perfectly possible. A few thousand people sit it? Not actually sure on that. The 1st year of biomed prepares you really well for the GAMSAT, whilst science would require you to pick the right subjects.
If you don't get into postgraduate medicine/dent/etc., your major options are probably to go into research or academia? It really depends on the person.
Overall, if you do a science degree and pick the right major/subjects you're just as capable of getting into postgrad than your equivalent in BBiomed.
My understanding is that you can do exactly the same subjects in BSc as in BBiomedSc... so it really doesn't matter if you plan to do postgrad medicine
A lot of the subjects in the BBiomed are restricted to students in that degree, so you have to do equivalents (for some there are no equivalents though).