Just for the fun of it, I'm going to make a pretty bold prediction; I'm expecting Xbox One to have a marketshare of roughly ~55 million by around ~2019 which would be ~6.5 years after launch. I think at the same time in its lifespan, both the PS3 and Xbox360 had both roughly 80 million and the original Xbox achieved around 25 million. I am predicting Xbox One to be somewhere in between the first Xbox and the very successful X360. Given these numbers and that the market is roughly split between the PS and the Xbox, I'm estimating that the PS4 will be roughly ~105 million. If my numbers are way off; I'm pretty much expecting a 66% marketshare in favour of the PS4.
Why? I think the PS4 has the momentum and the hardware to keep it ahead in all major regions. Europe is a strong market for the PlayStation brand and in NA, I think the PS4 will be ahead, although by a closer margin given the Xbox's strength. I think brand name recognition, loyal fans and an agressive pricing on behalf of Microsoft will ensure that the Xbox One will be reasonable competitive especially in the NA market.
I'll make a mental note to remember this post in 5 years to see how close or off it turns out.