You should opt Physics. I opted it in my first attempt in CE-2016. Part 1: 69, Part 2: 39. I don't know what disaster happened in the Part 2, probably I made some huge blunders, but I still maintain it is so easy to prepare and get marks in Physics for Engineers than to opt some Social sciences' subject.

In my other post, I have shared my Physics, hand-written notes will all concepts explains, formulas etc derived - a piece of cake for anyone having a Engineering degree especially Electrical, Mechanical or Civil.

@Madeeha Malik -> You better analyse the syllabus of Physics and see it for yourself if you can manage studying all those unbeknownst topics. And certainly this is far more advanced than F.Sc Physics. On a side note, you should opt CS. I have 145+ plus marks in CE-2016 CS paper - anyone with a CS background should go for it.