From your question it seems that you already know the answer. If you go with NATS you will be centralised into one of its new centres, this makes financial sense but puts all the eggs in one basket. You may find in a couple of years that your present radar atcos wont be required as the NATS centre will amalgamate your airfield with others and produce controllers capable of doing both jobs. SERCO on the other hand would probably stay at the site but in the long run would cost more money as they would treat it as an individual concern and need to train and recruit.