The 950XL is great, the OS is just okay.
I'd keep the Nexus and run with the XL for a few weeks if you can. That's what I've been doing (not sure for how much longer).
Brief background: I used Windows Phone 7 on an HTC and Windows Phone 8 on a Lumia. I always liked Windows Phone (i.e. the look and feel of the OS), but the app gap was too severe and the hardware too limited. When they announced Windows 10 Mobile, the new 950s and universal apps, I got excited again.
If I'm brutally honest, I'm disappointed. The situation with W10M is exactly like WP7/8 -- beautiful UI with lots of potential, but an unfinished OS with missing apps.
(All these years later and Phonos is still the only half decent Sonos app!)
In the few short weeks I've had the phone, there have been several occasions where I was simply unable to do something on my W10M phone that would have been trivial and routine on my Android phone. Sometimes due to missing apps (specific banking, email or other apps) or because apps are buggy / feature incomplete / have not been updated for W10M.
Sometimes really simple tasks can be difficult. Yesterday, for example, I was trying to copy payment details from one web page to another. (Ordinarily I'd copy them to a banking app, but I can forgive the missing banking app for now.) Inexplicably I was unable to copy the details -- the only option was "Select All", being all the text on the page. I checked on my Android phone and I had no problem selecting the details, so I have to blame MS and their Edge browser. "Fine", I thought, "I'll memorise the details and enter them manually". Except switching back and forth between web pages (to read and memorise and then enter the text) forced each page to refresh, losing any details I had previously entered! I actually had to write them down (I know, first world problems, but little things like this make W10M painful to use at times).
It's fair to say that most apps in the store need to be updated for W10M. It would be really helpful to see when an app was last updated, but this information doesn't appear anywhere in the store. "New and rising" is handy, but where is sort by date of publication? I did find a podcast app that seems to have been updated for W10M (called "Cast") and it works very well, which is a good sign. I have to imagine the app situation will improve, but equally I have been here before and it didn't end well.
Like I said, like the XL and prefer the look/feel of Windows to Android, but not sure how much longer I'll hold out. I'm not sure I can forgo the apps/functionality I enjoy on Android.