Hiya Natasha,
If you're successful at Stage 3 and pass the medical, you'll be asked if you'd prefer to go on the Aerodrome or Area course. There's no guarantee that you'll get what you ask for, but most people do. It tends to be Aerodrome is asked for more often, but has fewer places on the course, so you could say that one is more in demand, and it just depends how many spaces are left on the next courses when you pass.
After that, once you're at the college, the Aerodrome people will eventually get split into those for London Airports or those who need to do an approach course too (this means you'll go to a regional airport or Swanwick). This depends on what vacancies there are at the time, and so what training is required for those vacancies. It's the same course for everyone for Area. You can express a preference about where you'd like to end up, but business need must come first (you'll hear a lot of this).
Hope that helps a bit...