I got 11 A* at GCSE and have no idea what I want to be, I just want to keep options open.
They're all faciliating subjects and will open a lot of doors to you; it all boils down to whether you prefer/are better at Chemistry or Biology. Remember, though, if you want to go down the Medicine/Vet Med/Dentistry/Pharmacy route, you will probably need to have studied both Biology and Chemistry to A level!