It's true that Biology would open a lot of doors in medical fields, but maths is the subject that is advantageous to have for pretty much any degree.
If you're sure you want to go down a route such as medicine then I'd suggest biology. But don't rule out maths on the pretense that it might not open any doors.
So I suggest making sure you know what kind of degree you'd like to take before making the decision. Mind you, I thought I was going to do medicine and picked Maths, Chem, Bio and Philosophy. But realised I hate it and dropped biology and am applying for PPE.
Hope this helps answer your question.