This topic has already repeatedly been discussed. The crux of the discussion is that one should opt the medium through which one can explain one's answer in a befitting manner. I personally tried both the mediums and scored better when i attempted the paper in English. However, there were obvious personal reasons behind scoring better when I attempted in English.

• My Urdu handwriting is very pathetic .
  My Urdu Handwriting is very slow and you know where one has to attempt four questions in remaining papers , in Islamiat one has to answer five questions.
  The time I attempted in Islamiat in english that year the objective part was very easy .

So , if you have prepared the paper well and you dont have the issues i had , you may go with English. Whether , you go with English or Urdu , you will get same treatment from the examiner.
The one liner answer is that its not medium which has to determine your marks, but your answer and your presentation have to be rewarded.

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Thanks Allah