If it were me in your shoes and had to pick one, biology would be the go.
It may be easier to pick up biology in a school context than later on, should your interest take you there. There are many easy avenues to pursue philosophy at uni or through other means, but not so many for biology streams. Don't get me wrong, I didn't do biology, but did do VCE philosophy. I enjoyed it and would recommend you pursue your interest in it. Just sayin' they both have their merits, and biology could provide some balance to the subjects you have chosen so far.
Also, if you really think about what you learn in biology, it could change your worldview as much as philosophy would. Biological systems
are integrated into the world, after all. Aristotle was a biologist of sorts, as was Descartes, so there's no reason you couldn't see it as augmenting your future studies in philosophy.
There's also nothing to stop you reading philosophy on your own or with friends doing the subject, and discussing it.
while you're making up your mind, you might want to browse through the archive of these podcasts on philosophy and see what grabs your attention:
http://www.abc.net.au/rn/philosopherszone/index/date2011.htm#http://philosophybites.com/past_programmes.html