Hi Andy,
Ive just recently passed my stage 3 interview and will be going for my medical within the next couple of weeks. I also visited Prestwick ACC and Glasgow Tower. I felt it was really important to visit both as the disciplines can be quite different.
The reason I am telling you this is because one of the first questions the ATCO will ask you is "How did you prepare for this interview?". After I told him I had visited both a tower and an ACC, he seemed really interested and the conversation flowed naturally after that.
A couple of guys who were interviewed at the same time as me had only visited one place instead of both a tower and ACC and I know that they were unsuccessful. I know that the whole interview is not based around how many visits you make, that would be naive, but I think if by visiting both, it means that you will be able to describe your experience of both instead of only being able to talk about an ACC for example.
Best of luck anyway.
Any questions, just give me a shout
Kev