Hi Kashif,
Though both mediums are good for Islamiyat but I prefer Urdu for a reason. As we quote verses and hadiths, we spend more time in thinking peculiar words for quoting the translation of the verses.
But with the Urdu language, we don't face such difficulty and fluently we can attempt all the questions.
We gain marks by our systematic and comprehensive approach to the topic, not by the language.
Hope it helps.
All the best.

Thanks