I couldn't get fussy over a free phone, but if I had to choose it would have to be the 950XL.
The bigger screen size is my kinda thing, it'd be perfect for videos, games, and viewing pictures taken on that awesome camera. Aesthetically, I think it looks nicer, more unique, and the camera sensor design is a nice throwback to my trusty 1020 that I'm now using. It's a phone that looks like it packs some serious tech under the hood. Spec-wise, the faster processor makes it just that little bit more future-proof, and the bigger battery is always a bonus. Plus, liquid cooling. That just sounds epic.
That's coupled with all the reasons to want any of the 950 series, of course; Continuum would be great for my studies, the high-end camera is a massive point of appeal, and it offers the very best of Windows 10 on a phone.
Still, I would not turn down a 950 if offered!
Thank you for your consideration