I don't agree with that at all. Being passionate about your subject does not mean being passionate exclusively about that subject. I got in for history, depsite doing mainly science A-levels (Maths, Further Maths, Physics). Up to end of the summer I considered doing Maths, Physics, ASNAC and history. Four months before applying you SHOULD be considering all you're options.
Oh, and, I've no idea about the differences in the courses. Oxford has a compulsary thesis, which Cambridge might not. Do the research yourself: chances are the particular papers which appeal to you will not appeal to everyone else. Once you've made up your own first impression, then it's helpful to ask current students for more detailed. But definitely form your own first impressions from your own research, not others' experience.