It doesn't matter if you attempt Islamiat paper in Urdu or English. Plus point to go for English is that you have 11 subjects which are in English and you already have the pace in English writing. I think there is no other difference.
and if you attempt in Urdu, it would be easy for remembering ayat and ahadees, because verses should be copied exactly.
lastly, examiners will never give marks on attempting the paper in English or in urdu. As, FPSC has given this option.
rest is upon you thanks