Do you think it makes a difference for an applicant's chances for admission at Oxford Univ to state that one would like to take classes or specialize in jurisprudence instead of, eg, European law? Just because Oxford is "the" place for jurisprudence (and because, therefore, competition might be tougher). Put generally, do you think that one's specific preferences regarding MJur subjects have a considerable influence in the selection process?

I dont think individual preferences make a difference I believe you oughtta stick to what youd like to study. If its jurisprudence, fair enough, but if you like commercial law, human rights or anything else, it really shouldnt matter.

Just make sure that the university offers the courses you want and that you really explain why you want to take that specific course. I think that the most important thing is to have your application in line with your previous experiences and achievements.