This may not be helpful to you, but in terms of competition: applicant numbers tend to see-saw between Magdalen and Christ Church year upon year for the more popular subjects like Law and PPE because of all the people looking at the applicant : offer ratio grids in the back of the prospectus. One year there'll be a glut of Christ Church applicants in one subject, the next year in Magdalen.
For Law, the 2005 application cycle (the most recent) was a 'Magdalen year', which meant only 30 applicants were interviewed at Christ Church for 10 places (at odds with the usual pan-college 5:1 ratio for that subject - and many had been pooled from Magdalen), suggesting that the pendulum effect will lead to larger numbers applying there next year. Therefore Magdalen may be a better bet.
Sorry, that was incredibly poorly explained.