Hello Mike,
I can't offer much in regard to the 1520 however I must say that I am enjoying my 950 xl thus far. I will be honest and brief with my own findings and hopefully it will help you towards making an educated choice.
The display is beautiful, unarguably so, but it gobbles up battery life. For myself I have found using darker colored themes and backgrounds while keeping brightness around 50-60% (Still retina burning whites) helps to curb consumption.
The UI is very responsive but does have the occasional hangups. These are few and are not to the degree that it frustrates me. The Software was easy to navigate.
The camera is gorgeous but the software lacks some editing tools that'd I'd like to have. App for it perhaps?
Up to 200GB of additional storage was a big plus but currently only Sandisk offers such a Micro SD so I am waiting.
While smaller than your 1520 you shouldn't find the 5.7 inch display claustrophobic inducing.
I have experienced that reboot problem that has been mentioned on other threads and it appears to be related to my Wi-fi (unconfirmed). If I stick strictly with LTE I have yet to have the device reboot.
In short the few 'negatives' that I've encountered seem to be software related and I would imagine that Microsoft would address them. The battery life seems to be the most concerning issue for most. The 950 xl is an otherwise really nice device, seems built to last, and has potential. Hopefully more will chime in to give you better input than I but I do hope this has helped.