Do not know about other students but in my opinion Urdu is much better choice. I had prepared the exam in English. I was well prepared but when I saw the paper, I immediately decided to wrote in Urdu. It happened with a lot of other students as well. It was easy to write 4-8 pages on questions asked in CE-17 papers in Urdu. I felt it difficult to write in English on those topics. It is general perception that students who write in Urdu gets more marks even my teachers say so. I have always attempted Islamiyat in English but in this paper I do not know what happened suddenly I started writing in Urdu.
Also in Urdu it is easy to write verses as writing style of Urdu and Arabic is almost same while in Engish you have to write it in English. You are not allowed to quote verses in Arabic. Some students say that one can lose marks by Arabic verses. The best way is to write Quranic verses and Ahadith without putting "Araabs" on them. This way no one will deduct your marks as originally there were no "Araabs" on Quranic verses.