I am from a Catholic school, and our SOR II classes did Christianity, Judaism and Islam. It was split pretty evenly between Christianity and Buddhism for Section III, although basically almost all of the people who did Christianity (I'm talking about the exclusion of 1 or 2 out of the 30 people that do the subject in our school) found themselves unable to understand the questions for either Christianity or Judaism., and so just picked Christianity (seeing as they did Buddhism in Section II) and tried to apply their practices, ethics or significant person to the question. And no-one in our grade came preparing to do the Buddhism question, but those that ended up doing it only chose it because we couldn't make sense of how to apply our knowledge to Christianity and Judaism, and we found the Buddhism question to be easier and more understandable to answer.
So, in saying that, I do think we felt unprepared for Christianity, and Judaism for that matter, but, we pretty much sucked it in because we chose the question that was answerable.