FIrst things first:
1. Both are produced by Microsoft, which is the 1st company in technology with new tech devices (like: hololens , surface, windows, phones, etc.)
2. One of the most stable OS
3. I love what Microsoft is doing and for that i will always buy a windows phone
Now to compare the two devices:
In my opinion both are good, it's hard to decide which one I would choose, but i will try:
And i choose Lumia 950 XL, why?
Because:
1. It's bigger and i love big phones.
2. The performance is a little bigger.
3. I like the design.
4. You can use it as an computer.
5. Big performance for a windows phone.
And many others.
I'm a big windows fan and i hope to see more new things created by microsoft.
Keep it going devs, you know what you are doing.
With Respect,
Andrei