i whole heartedly believe that the X1 will struggle to surpass the WiiU in lifetime sales, the X1 sales seem to be slowing right now while the WiiU seems to be hitting a silver medal worthy stride. week in and week out for the last 3 or more months the WiiU has outsold the X1 and thats without any major games past MK8 as of late.
The problem i forsee is the gamers who want the multiplat gamers are going to the ps4, and then you have the gamers who want nintendo games, this leaves the X1 out right now, while im sure games like Halo and Gears of War will bring thier own amount of players to the X1 i do believe as with Titanfall(but to a bigger extent) this will be short lived. What i mean is Halo will bring players over for about 2 months IMO same with Gears after that 2 month period the gamers who want to play those will have or already had a X1.