Hi, I do both!
these two being some of my fav subjects i am ofcourse bias but...
geography is obviously great, if you enjoy it, you will inevitably enjoy the syllabus. yes, its a BIG jump form GCSE's as the questions are longer and harder. but i think overall this has benefited me with essay writing and just thinking about things from a geographical point of view, rather than just sciences and maths which are my other subjects. regarding taking a subject to go with geography, don't worry about that. NEVER take a subject you don't enjoy just because you think it will be helpful.
chemistry is damn hard but also fun. its necessary if you want to do a medical course at university but also is a great subject to learn. not to mention everyone seems wowed when they find out you do chem. namely because its the hardest a level and people look up to that.
from either you will learn new ways of thinking and learning. chemistry has widened my knowledge so so much, so ha geography, but from geography i also learnt new ways to answer questions etc.
finally,
id say do both. haha. but if not, you have to think about which one you would enjoy the most and can see yourself doing for two years. look at some a level text books so you aren't disillusioned.
Hope that helps.