English is a better medium nowadays, as a lot of questions put forward are related to current affairs. and we can communicate them better in English, as we prepare for them. But still, the quality of the material is above all these notions!
Focus on content first. Do choose a medium in which you feel at comfort and fluent in writing. I did my 2011 Islamyat in English and secured 61 marks, although I had studied Islamyat in Urdu upto H.S.S.C.