It's an interesting question and I don't really have the right experience to offer an opinion - all I would say is that there's a lot of nonsense spoken in NATS (which appears to start at the top) about SERCO.
What I find strange is that the powers that be seem to be making this decision because they are facing an imminent shortage of controllers. If this is the reason behind it, it's a very short sighted policy. If they are not careful, as jack-oh points out, the ATC unit could be taken to a state from which it will be almost impossible to return the situation you have today.
Why can't the unit attract new staff - yes there's a shortage of controllers (and it looks like it's real this time)? Presumably because it doesn't offer attractive T&C, or maybe the unit has a reputation to be avoided (because of the way that staff have been treated in the past). If there are problems today, they will almost certainly be exacerbated by the use of contract staff.
From an individual's point of view, does it really matter who gets the contract? As a controller you can't do much to influence the decision and in the end it will be made by accountants who won't understand the operational aspects anyway.
With one or two exceptions, controlling in the UK these days is a matter of keeping your head down, taking the money and stop thinking 10 years out of date about loyalty to the unit or serving the aircraft operator's needs.
Shame really.