I'd choose the Lumia 950xl, as the x3 has less features aside from their premium remote emulation service.
The X3 is a great phone, however it doesn't have Glance, or any of the other Nokia exclusive features like equalizer (for audiophiles really) that we are used to seeing on windows mobile handsets. There is a list out there about the features missing from this phone in one of these threads on this site. To some, these features are negligible and they couldn't care for them. For others like myself, the lack of those features shied me away from buying a more expensive phone for solely more hardware. Keep in mind, compared to he 950XL, the camera on the x3 is abysmal. Perhaps it's not noticeable in greatly/well lit conditions, but as soon as it hits evening time, the difference in quality Is quite telling. Also, the double tap to wake on this phone is the most inconsistent "feature". It's borderline broken. Another issue that may be a deal breaker, Tap to pay is NOT available on this phone.
I'll list the only advantages the X3 has over the 950XL:
*Apparently doesn't get as hot as the Lumia 950xl because of the processor used. However, people also fail to realize that there is a software/firmware issue at play here. There have been threads popping up about "super hot" X3's with massive battery drain (issues that also exist on the Lumia 950xl)
*Much better battery life, by a significant margin.
*Fingerprint scanner
*People claim the IRIS scanner is quicker on the X3, however from my own experience, the time difference between the two is minimal, if at all. You be the judge.
*The X3 is *seemingly* faster than the Lumia 950xl under use, however it has a slightly smoother Continuum experience.
*Exterior Speakers - I don't care about this. However the speakers on the X3 are far better.
The X3 is a "business/enterprise oriented" device, which in itself is used as an excuse by many HP fans, however the argument doesn't exactly hold up, when looking a little deeper as to what exactly "enterprise oriented device" really means. Missing features are missing features, consumer, prosumer, or enterprise.
Do plenty of your own research before pulling the trigger. Anyone (including HP) who tells you this phone is "feature packed" aren't exactly being honest when compared to what the Lumia line offered.