I don't think there is a chance in hell that the Wii or the 360 will sell 20m units by the end of this year. Remember we are 1/4 of the way through already.
I predict that the Wii will be beating the 360 by 1m come the end of the year with Wii sales on around 16m and 360 on around 15m.
The introduction of the PS3 will dent some sales of both systems, but more so the 360, but I don't expect to see higher sales of the PS3 until a pricedrop is introduced.
I reckon the Wii and 360 sales will be level around October/November time with a better Christmas for Wii taking them 1m over 360 sales.
This means that MS have 8 months to sell another 4.5m 360's and Nintendo have 8 months to sell 9.5m units, which is alot.
I cannot see either console selling more than what I have predicted...being more realistic, it's likely to be slightly lower than that.
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)
