I'd go for Maths, simply because there are an enormous number of resources available and the content is structured in a very clear, procedural fashion. Personally I did 6 A-levels (self-studying 2), and the amount of time you need to sacrifice is significant, especially if you're planning on taking the whole A-level in one year. It's worth it if you enjoy a challenge, and it can be a talking point in future job interviews. Maths will also be especially useful if you want to apply for any job requiring numerical competence in future, e.g. in the financial services: having an A-level Maths (especially if it's an A*) will really help your case.