WexCan,
Area (and London airports) controllers can earn up to 50% more than most regional airport controllers.
As to the streaming process. You are told whether you'll go Area, Aerodrome (Tower) or Approach about half way through the basic course. These days the vast majority are streamed Area. How the decision is made as to who (if anyone) is streamed for Airports ... as a Trainee you'll never really know! None of my course could work it out.
Regards, ADIS