I'd choose the 950 XL over the 950 for a couple of reasons:
1) Bigger screen (but not too big like the 1520 in my opinion), for media consumption, Map viewing in car, not to mention the added size for keyboard is pretty damn awesome.
2) Faster Processor.
3) Bigger battery.
I'd choose the 950 XL in general over other phones (and OSes) too for reasons below:
1) Continuum.
2) It's a Windows 10 flagship with lots of potential and seems stable enough now (Been using nothing but windows since winmo 6.1).
3) Nice and thin and not too big.
4) Cortana is more useful than the other assistants (I use 'Hey Cortana' a lot in the car).
5) The UI is nice and neat and highly adjustable.
6) In this context, it's FREE. Who doesn't want free stuff, especially a phone that meets my needs?
7) I can ****** earlier for real without having to wait for my Lumia 930 contract to run out in 2017 to get the 950XL.
8) More responsive than most Android, more functions than iOS.
9) If the experience is not good due to early OS some bugs (let's not kid ourselves, the two flagships have some for now), I can still whinge on Windows Feedback, and actually get most of my issues considered and fixed.
10) Once the big bugs are fixed, I can actually show off the phone and maybe convert a few friends to Win 10.