The current breakdown of numbers of students going through Valley is approx 4-5 RAF and 1-2 RN per course.
It is a known fact that the RN need to get single-seat scores throughout Valley just to stay there. Meaning an RAF chap may get through on 2's, 3's and 4's whereas a RN pilot will need more along the line of 4's and 5's.
The RN is also no longer sending it's marginal pilots to fly twin-seat for a year or so to improve as there just aren't the seats.
IMHO looking at the above it can only mean you have a better chance of climbing into a FJ cockpit if you go the RAF route. Simply because there are more jets and some of them are twin-seat.
Hope this helps.
TF