As I see it:
Microsoft and Sony will be fighting it out over the next 2 years. Microsoft will be fighting it's hardest to maintain it's current market share, while Sony requires to eat into their marketshare. Nintendo on the other hand will quietly be pushing up and up.
Microsoft have to maintain their NA lead and have good solid sales in the OR. Japan is a lost cause to them. On the other hand, Sony have to bring some fight to the US, continue to grow, even if it's a reasonably slow rate in Japan and get stronger in OR.
It's the Japanese factor that I see being decisive in this fight. I'm sure around 25m home consoles were bought last generation in Japan. Currently only around 5m have been bought this generation and if Sony can get at least a third of these sales with the Wii winning the Japan. This would account for a solid 8m sales in Japan. This 8m will make the difference. But Sony need to be pushing in all regions to break into 2nd spot and I don't see this happening until the end of 2009 at least.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)