I don't know if anyone has suggested this, but best thing is to do work experience in both and see which you prefer. With IB, you have to take a mixture of subjects right?
So either way you'd be able to apply to either law or medicine when the time comes for you to decide. With Law you don't need specific subjects for uni (unlike medicine), but either way with IB due to the fact you have to take what is it 6 or 7? you'll be covered anyways.
And then decide on which you genuinely enjoy, I know people who are doctors who say the pay isn't that great for the job they do BUT they love it and wouldn't change it for the world. I also know some who have actually dropped out and switched to law! I also know of some lawyers who after 8 years at a magic circle firm are getting bored and others who love it!
So really depends on if you like the work or not. So really, just do some experience, even if you can't get work exp in a hops, do it in a healthcare environment and see if you like the feel etc. With both fields, there is a chance to switch the line of work you're in if you really wanted to and so there will always be work (for example going from a commercial lawyer to a family lawyer, or from a A+E dr to a cardiologist etc).
Either way, good luck!