My eldest son happens to be really good at both Spanish and French (and fairly horrible at most other subjects so don't think I'm bragging) so we were also torn between which one he should choose for his GSCE. In the end, we decided that while I have a soft spot for French, I haven't really used it since I left school nearly three decades ago (eeekk), while Spanish is the second most widely-spoken language in the world and is therefore more likely to be useful in the long run. Ultimately, however, it should probably just come down to whichever language they prefer as the more they enjoy a subject the easier to will be to learn and do well in.
