It's about getting the balance between what you're good at and what you enjoy.
If you're a mathematical genius who is getting 100% on GCSE papers with no effort whatsoever, and Spanish makes you want to cry whenever you think about it because it's so difficult, then Maths is your best bet.
However, if you're only marginally better at Maths than Spanish, and you actually really enjoy Spanish, then go for Spanish. It's really hard to concentrate in a lesson you don't like.