Really up to you. I'd suggest looking at the syllabuses and seeing what is covered in each to see if the particular topics interest you - when I was doing my GCSEs and picking options, for example, GCSE History was referred to as "Hitler studies" because the emphasis was pretty much purely on World War II Germany. A lot of people might not be as interested in that, potentially. Of course that was quite a long time ago so the syllabus has probably changed, but the principle of seeing what is covered remains and is worth considering.
You may also want to consider if your school requires you to have done either to GCSE to continue to the A-level, and whether the A-level is necessary or useful for you in future. Particularly if you want to apply to uni, history degrees normally require A-level History. Geography degrees don't always require A-level Geography but it's required about half the time. It's also useful background potentially for earth sciences/geology at uni I gather.