Natasha,
At the moment, it sounds like people are being given a choice as far as possible before starting. However there has always been a higher requirement in the company for area controllers vs. aerodrome/approach controllers. When I went through several years back, we were offered the chance to state our preference after we had started at the College, but there were always many more folk wanting to do aerodrome/approach than places available, so people were frequently disappointed.
They will try and take your choice into account, but ultimately the business need will take priority. Incidentally, this applies equally to unit postings once you have finished at the college - you can say where you'd like to go and they will try to take that into account, but you will be sent where there is a requirement for trainees.
Basically, be adaptable and make the best of whatever you get, even if it wasn't what you wanted. It's not a job for people who aren't prepared to be sent to the other end of the country from where they'd hoped!