According to the details shared by World of Tanks developer Wargaming creative director and executive producer TJ Wagner, by 2019, PlayStation 4 and Xbox One combined sales (worldwide) will be 108 million units, with PlayStation 4 leading the way with 69M units sold. These figures were first made public at Game Developers Conference presentation (slide) by TJ Wagner.
Another slide from the presentation stated that currently there are 36 million PlayStation 4 user base, and just 19 million Xbox One. Please note, these figures came to Wargaming from different (but not the official) sources Superdata, IDG Consulting and IHS.
So far we have received official word only from Sony on PlayStation 4 worldwide sales. In January 2016, Sony revealed that PlayStation 4 has sold nearly 36 Million units worldwide. On the other side, Microsoft has not yet a single word about Xbox One worldwide sales. According to reports from different analyst, PS4 has a 2:1 lead over Xbox One and so in January 2016, it was estimated that Xbox One sales are around 18 million mark.
You can check out the full presentation of Wargaming GDC Presentations here.