Philosophy and Biology are my two largest passions in life so i thought i would chime in here.
You can do philosophy outside of school, by yourself, relatively easily. You'll still be able to learn rather well just buying books and reading them. You might miss out on the discussions you would get in class and writing about it but you can just hit up a forum or two and practice it out there.
Biology would be much harder to learn on your own i think.
To be honest, i chose biology randomly in year 10. I needed one extra subject to "fill up" the 6th spot i had. It was either biology or geography. I ended up choosing biology, it was one of the best decisions of my life. I really came to appreciate and love the subject. I didn't think i would like it at all but my mind slowly changed. It totally changed the direction i was going to take in life and its just plain so damn amazing.
So, just on that basis, i think you should pick up biology over philosophy.
If you got any questions (or anyone else) feel free to ask me here or in PM.
(Note: I didn't undertake VCE philosophy. I did study it in my spare time and later pick it up at uni but i don't have much experience in the VCE course. I did undertake VCE bio though.)