I did Classical Civilisation and found it to be very easy. As a fourth subject I found it to require fairly little effort compared to the effort made by my friends doing English. Since you already will have a lot to do with History and Spanish (I have no experience with Philosophy so I don't know what it's like), I would recommend choosing CC because I think that it may be less stressful yet still be a good example of further essay writing and analytical skills. But, at the end of the day, it's whichever one you think that you would enjoy more, and it also depends on what you want to do at university.