Right now, Wii U is behind the GameCube. I think it will pass it up eventually. Let's look at Nintendo's numbers for each console.
The left year will be GameCube/right will be Wii U
FY 2002/2013 GCN- 3.80m / Wii U- 3.45m
FY 2003/2014 GCN- 9.55m / Wii U- 6.17m Growth GCN- 5.76m / Wii U- 2.72m
June 30 ending 2003/2014 GCN- 9.63m / Wii U- 6.68m Growth GCN- 80,000 / Wii U- 510,000
Now lets look at future GameCube numbers... left will be LT numbers and right will be growth.
September 30th 2003- 10.45m/ 0.81m (price cut to $99.99)
December 31st 2003- 13.94m/ 3.5m
Now end of year numbers
December 31st 2004- 18.03m/ 4.09m
December 31st 2005- 20.61m/ 2.58m
December 31st 2006- 21.52m/ 0.91m
December 31st 2007- 21.74m/ 0.22m
Here's what I think Wii U numbers will be
September 30th 2014- 7.29m/ 0.61m
December 31st 2014- 10.73m/ 3.44m
Now end of year numbers.. I think Wii U will have a 2015 peak, 2016 similar to 2014, and 2017 to begin the true fall of Wii U sales
December 31st 2015- 16.80m/ 6.07m
December 31st 2016- 21.69m/ 4.89m
December 31st 2017- 24.09m/ 2.40m
December 31st 2018- 25.45m/ 1.36m (release of new console)
December 31st 2019- 26.02m/ 0.57m (probably last year of Wii U)
My prediction may change in the future but based on current sales.. this is what seems likely. I will make another one of these in late 2014/early 2015 if I remember.. So I expect Wii U to pass GameCube sometime in 2016
POST YOUR THOUGHTS AND YOUR OWN PREDICTIONS IN THE COMMENTS BELOW AND VOTE THE POLL
EDIT: Even if 2015 isn't Wii U's peak year it will still pass up GameCube. 2014 has MK and SSB and other 3rd party games...
2015 has fucking ZELDA, Splatoon, Star Fox, more Nintendo games, Xenoblade Chronicles X, and more
So even if 2015 isn't the peak year it will be about the same as 2014. The only reason 2014 sales are really low is because the first half of the year was abysmal (almost as bad as 2013) now it has a new baseline thats twice the size of the old one. Next years baseline will be at least 50k and most likely 60k-70k. Now Wii U is even beating Xbox One weekly after Xbox One's pricecut. If you say because Mario Kart 8... MK8 gave it a permanent boost.. not a temporary boost like people were saying. Staying around 60k for over a month now. Didn't go back to sub 30k like people said.
Just making a point that even if 2015 isn't the peak year it will still most likely outsell GameCube.