Privatisation vs Nationalisation.

What criteria do you believe decides whether something should be Privatised or Nationalised (or otherwise)? I have a few ideas, but to ensure no one is biased against the OP turning the thread into a flame war I will post as anonymous later once my IP rotates.

Is it a benefit or a burden?  Either way, really. 

For privatisation, the question is: can money be made from this in a way that will not degrade the quality of service? 

For nationalisation, the question is: is doing so vital to the interests of the nation and of the people within it?  Is the current state of affairs causing unmitigated harm?  Does the government have to get involved?