I'll pick French. This is because universities are now demanding qualifications at languages, and if you don't study a language at GCSE level, you'll be at a big disadvantage when it comes to apply for unis. For many schools. languages are compulsory (such as my school), because with a language, your choices are quite limited.
However, this doesn't apply if you are already going to do a language at GCSE and you were just considering French as an extra language.