Ok here are the ups and downs:
Assuming the Surface Phone is true and comes out, it will probably be (rumors) a ~6inch phone/tablet device with Surface pen support. Assuming all rumors are true, it will be a Intel Atom based, and have the ability to run Win32 programs under continuum. Resolution should be the same as the 950XL, as the Intel Atom CPU integrated graphics can only do so much (keep in mind it will need to drive 2 display for continuum). It would be impossible to run Win32 programs on your phone screen because I can assure you that 99.99% of them would have its interface unusable.
Intel's latest and great Atom should be faster than the CPU in the Lumia 950XL. Not to mention that Microsoft is master at optimizing code for x86-64 architecture as this is what they work on the most.
By the time the Surface Phone is released, Windows 10 Mobile should be FAR more polished, and would feel like a solid polished OS (I mean it would be released end of 2016 is anything, so yea, it is to be expected).
But... you'll be waiting ~1 year for it, and that is assuming it doesn't sell out, and you are able to get your hands on it at release, let alone even exists.
As most reviews of the Lumia 950 series is that they hated the back plate of the device, because "it doesn't use any premium materials", not getting that you need plastic back to have wireless charging, and replace the battery, expect those feature to disappear (unless Microsoft figures something out, which could be possible, as we seen the Surface team is very talented, and have been pulling the impossible a few times in a row), possibly the micro-SD card expansion as well. Although, they might do like Apple with the iPhone 6S Plus 128GB model, where they put an SSD (and this is what REALLY helps the iPhone to be faster). If they use an SSD, expect the price to skyrocket, because SSD of 64GB+ in 1 or 2 chips only, is very costly still (although price is dropping as technology moves forward).
Here is my thought:
-> Surface Phone is real
-> Lumia 950 and 950 XL are test platforms for the Surface Phone. What I mean is notice the different button layout, they are trying on us which button layout is better. The 950XL has a heatsink solution on a hot processor, great for testing out the ability to cool off a powerful Atom CPU (or maybe Core M CPU)
-> The name "Surface" is very strong in North America, "Lumia" is non existent there. So, just the name will push sales of the device. But Microsoft needs to make sure that you have at least the core apps for most people (and that Microsoft needs time, and continue to encourage devs to make Universal Apps), and have an OS polished.
-> Chances are that the Surface Phone will be very well build, and feel high quality as the team is really good on this, and so far, doesn't seem to want to settle for anything else lower in terms of quality. Basically no cutting, despite loosing out on the ability to put better parts (see Surface 3 non-pro).
Those 2 last points alone with more polished Windows 10 Mobile OS, and more apps, will push more favorable, even glowing reviews, which will build further hype and push sales. You might feel disappointed when the times comes if you buy the Lumia 950/950 XL now.
But, on the other side:
-> If you are happy with Windows Phone 8 / Windows 10 Mobile now, you should be fine.
-> You get to enjoy a product early, despite not being perfect at the moment.
If you can wait, and live with your phone for 1 year to a year and half, then wait.
If you can't and want to continue with Windows Phone, then get the Lumia 950/950XL