I think MS will continue to lead in the USA, but with launching the same time, and with PS4 being cheaper it has a good chance of at least keeping up with it. UK will also be a lot closer, while i think the rest of Europe will certainly move more towards Sony again (price and being more inclined to buying a Sony product).
I suspect Xbox will have trouble in Asia, especially Japan, which will provide more benefit to Sony as it did this generation.
So overall, as long as Sony don't screw up, they have a chance of creating a more significant gap than this generation, perhaps 10-20 million? It's really difficult to predict, because we really dont know what dumb move either might make and cause issues.
I'm hoping a smooth launch for both consoles, and that it is decided by games and features, rather than PR guys making a big mess of things.