We all know the mainstream differences, so I'll spare the recap and just focus on the major differences...
I owned the Moto X 1st gen, 2nd gen, and now 3rd gen.
An android phone is an android phone unless it provides better AI(Artificial Intelligence).
But Here are the primary reasons I love the Moto X line: (which Motorola patented, and no one else has)
1) Moto Assist such as touchless wave to get notifications
2) Gestures: double twist for camera and double chop for flash light
3) Motion sensors on all 4 corners.
But I am seriously considering to switch to the Nexus 6P for the first time in 3yrs.
I too ordered the 6P BUT it must prove to be superior in ALL areas below: (otherwise, I'm staying with the X)
1) Sensor hub: Needs to wake upon touch (proves it can have AI against the X wave touchless). The blinking light is a plus.
2) Battery life: it has 450mA more, which is not huge but it is 1/6 more so that is substantial. It needs to live up to its 3450mA vs the 3000mA of X.
3) Double power button for quick camera on (to replace the double twist on the X)
4) 6P claims to have additional gestures (need to test to see what additional gestures are built in the 6P)
5) Camera: 1.5uM needs to prove it's superiority. (After all, 6P does not have front flash, which is a big negative. I didn't think the front flash was necessary until I needed to take a pic in low light condition with a dimm lit area and a background sign. My wife took a selfie with her Iphone 6(no flash) and I took a selfie with the X (with flash). The result was phenomenal, the Iphone was bright but super noisy. The X was perfect tone and balanced color, literally no noise.) In other words, the 6P needs to prove it's superiority against the X Pure at least in the no flash category(because 6P does not have front flash). Then it needs to come close against a front flash of the X.
6) Marshmellow better give better enhancements and longer battery life.
I will have to live without the double chop flash light, which is very convenience!
All in all, spec against spec is spec. Real life usage determine UI and ease of use.
In respect to looks, it's a matter of preference. I love the all metal feel but I also love the Moto maker to give the user the ability to choose.
I've chosen the wood back for the X 1st and 3rd gen, plastic for the 2nd gen, all are great.
The fingerprint sensor and LED notification light on the 6P are great additions.
That's all! Can't wait to see the Nexus 6P prove its superiority against the Moto X Pure!
All in all, price is pretty comparable so you can't go wrong with either but at this point I'm leaning on the 6P to prove itself.
Good luck!