Well CSS is federal exam held alike in all the provinces every year in February where as PCS is provincial exam held almost every four years for vacant positions for BPS-17 in different departments.
In css,major departments are 'Police,DMG,Foreign Services,Customs,Income tax etc' and in PCS People are hired as DDO revenue,Section Officer etc. A CSP or PSP would end up at BPS-22 but a PCS Officer would at most reach BPS-20.
Now as far as subjects are concerned,you should opt which you feel easy with.You are an M.A English but literature is a low scoring one as per a general opinion so go for some other ones i would suggest.
Regards