The main difference between PR and journalism is the stance each one takes on objectivity.
It seems that PR practitioners are not objective. The public expects that. If someone comes out and makes a statement for or about a given company or individual, we expect that information to be swayed. The opposite side of the story isn't given any light because that isn't why the PR person has a job.
Journalism at least claims to be more objective. It may not be and can be in fact more skewed but they at least make it a point to act like it is objective. Both sides of the story are to be presented for a good journalism piece.
I had a professor say once that if people were thinking about going into either journalism or PR, 'go into journalism. It's easier to get into PR after being in journalism that trying to get into journalism after practicing PR.' I thought that made sense because if you are making a point to be objective it would be hard to hire a person that made a career out of the opposite.