This was the reply given by Farhan kaif when I asked same above question in I.R thread.


If you're to opt only one of them and looking forward to know about a candid suggestion then I'm here to share my experiences with you. I would say please you to go for IR, physics is very unfaithful in CSS itinerary. It needs lots of efforts to have grasp on each and every topics of physics 100pc and numerous questions are always found undoable, I mean you won't be capable to reach to the exact answer of such questions. In short, Physics can fetch you top marks in very rare cases, I'm being Master in Physics, opted both of them in my CE-2018 attempt but did well in IR than my own background subject Physics. Yeah, if you've recently done Master in Physics and can solve out each and every unseen numerical equations, plus, also aware of all your Master course then don't let this opportunity to opt Physics. While on the flip side, IR is considered ever green subject and can obviously uplift you in World's politics, English Essay, CA, PA, US History and International law but Physics has no intersection with any other subject in CSS optionals at all. In this very subject, either you will do well or null because with every theory question there's attached one numerical question, whose solution is sometimes out of the approach, no matter whether you've done M. Phil in Physics.
P.s must solve out last 3 years physics papers before selecting it, if you can make it then must go for it otherwise stay away from your background subject, it has no mercy with its Master holders either.
GoOd LuCk

__________________
live like ALI (A S)
die like HUSSAIN (A S)