SEO Tip #128: Is Comma A Separator In A Title Tag?
Matt Cutts: The only thing that you need to know right now that sticks out and leaps to mind as a bad separator is the underscore. This is because you might have a_b and we treat that like a_b rather than the words a and b. So, spaces, commas, pipes, dashes, all those things are good separators.
So in general I would say try to make a title or a title that looks pretty for users and any one of those characters can act well as separators as well. So whatever you think looks best for users, whatever converts best, any one of those should work just fine as far as the SEO.