for gcse?
in my experience, spanish is more useful, interesting, and fun. it's also, given the amount of holidays people spend in spain, and the buits of language they pick up, easier.
french is essentially very similar, and at gcse you really do not need a love of any of the langauges. a few set phrases and a decent knowledge of vocab will get you an a/a*.
looking into the long term, i'd reckon spanish to be more useful in university/business, given it's popularity and spread. Though of course, languages even at this level are often embraced as equal.
so, if it were me:
if you just want to go to gcse, then leave it, whatever you prefer/are better at, so in this case french.
if you plan on making a career involving foreign travel, spanish may possibly be better.
i didn't take spanish, since i would have only had 3 years to learn it, instead of 5 for french. with hindsight, i could have done it easily, but never mind...