I would not recommend picking Computer Science. Although the allure of *possible* high marks is quite intoxicating but it is very risky. The 5 or 6 months which you will spend on learning just "one" subject, you'll be better off spending that time in 5 other subjects and prepare them good enough to definitely get better marks and recover the marks that you could have probably scored in Computer Science.
Also even if you have studied Computer Science (i did too) but the course is just too lengthy and simply too much. Besides learning Pol. Sci. or Int. Relations won't hurt and you might even find them interesting.
One more thing to consider is that CSS is atleast a three year commitment. Revising such lengthy course before papers each time and then on paper night will be very difficult.