In the UK, students rarely get to choose who provides their academic reference. It's written by the school, generally, and may include contributions from the student's form tutor, head of sixth form, individual teachers, etc.
UCAS recommend that your referee be "someone who knows you academically and can talk about your work ethic, interaction with other students and your suitability for higher education or a future career." So I would select whichever of the two teachers you feel would fit that description better.
If you really can't decide, then perhaps choose the Maths teacher as that subject is specifically requested by many more Computer Science courses than is Computer Science itself.