Lot of opinions floating around here, but here are the facts.
Sales Stats
-This year, the Wii U has sold 1.0M (VGC), and the Xbox One has sold 1.5M (VGC)
-That means that the XB1 has sold roughly 50% more than the Wii U consistantly this year.
-XB1 is curently behind the Wii U world wide by ~1.7M, but has already outsold it in NA (lifetime).
Deciding Factors for the rest of the year.
-XB1 will be avaliable without Kinect for $100 cheaper than it has been previously.
-XB1 will release in 20+ more countries later this year including, but not limited to Japan, China, India, & the rest of Europe.
-Both XB1 & Wii U have a solid line up of exclusives this year.
WII U: Bayonetta 2, Smash Bros., Toad Treasure Tracker, Hyrule Warriors, & Sonic Boom.
XB1: Sunset Overdrive, Forza Horizon 2, Halo MCC, Disiney Fantasia, Dance Central (not retail), Ori and The Blind Forest (Not Retail), & Project Spark.
-XB1 will also have a wide array of 3rd Party titles to help it out such as Destiny, AC Unity, FarCry4, GTA V, Minecraft, Evolve, CoD, Battlefield, Dragon Age Inquizition, LoTR: Shadow of Moridor, Alien Isolation, The Crew, ect.
EDIT: Also, I just wanted to mention that if you don't include how much the Wii U has sold in Japan (1.7M), it's lifetime salse would be equal to the X1 (4.5M).