I would have picked the 950XL. I would have done so because:
I prefer a large and rich screen.
While I currently use a 1GB phone with the latest build of W10M and it works fine which is proof that W10M is kind to lower spec'ed devices, having 3GB of RAM and a high-end processor would make the experience more fluid/ smooth and lightening quick.
Surprisingly, I like the 950XL's design language. To me it is an example of understated elegance - polycarbonate nothwithstanding. I think the revised version of the Start screen works very well with a device that sports the kind of display that the 950XL does.
The fact that the phone is light and has a removable battery are also plus points.
The prospect of Continuum and what it may evolve to over the course of next year is also one of the 950XL's attractions for me.
The 950XL's camera combined with its low-light capabilities (as compared to my last two phones - the 830 and the 640XL) and OIS will give me a fairly sophisticated point and shoot option. But to be fair, the cameras on the Lumia line of phones (in general) have been excellent and the 950XL probably extends that excellent reputation.
Lastly, I suspect that the 950XL is one of the last phones that will carry the design language that it does. Going forwards, I think with Panos heading the Hardware element of MS's business, the design will change. If speculations are to be believed, high-end MS phones will be similar to the Surface design language. If true, this makes the 950XL the last high-end phone to carry the classic Nokia design language (albeit in a slightly mutated form).