I am writing this late but I wish it can be helpful. I opted CS in 2015 and got 120+ in both. The paper was not too difficult nor too easy but when you already have prior subject command(degree) you can go for it.
The only reason i can give in favor of not opting it if you don't have programming background(job, not a good programmer) because in 2016 paper the examiner asked to write code( with proper classes, functions).

I studied from recommended books because I already had almost all.