I've worked at more than one Serco airport, and my current contract manager and manager ATS are both good eggs. Although the company has been through a few name changes, the area management side has remained mainly intact. I am reasonably well paid, and have loss of licence insurance and am in the pension scheme. It seems to be that if you do your job, take the money and keep your head below the parapet, things are fine. This is a much edited entry, so you may wish to read between the lines.
I didn't work operationally for NATS so I can't comment there, but from reading this site for the past few years, there seems to be proportionally equally as many disgruntled NATS chappies/chappesses as Serco.
You've a tough decision to make.
Best of luck.