The BIG difference between the kinect V1 (x360) and the kinect V2 (xONE) is that the V2 has a better camera resolution. Because of this it is able to track more bones. So if you want the best tracking the V2 is better.
BUT the reason why most people use the V1 is because of its price tag. Also from my personal experience the difference between the tracking is not worth the difference in price. The kinect V1 tracks the major bones just as fine as the kinect V2.
Conclusion: go for the kinect v1, its tracking is good enough for most games and it is way cheaper.