Re: MXPE 16GB owners - is it enough?
New guy here. Just picked up a Moto X Pure Edition, 16gb for $350 at a big box store ($399 for the device and a $50 gift card). As context, I used an HTC One M8, 32gb internal plus 32 gb expanded, from April 2014 until last week. For two years prior to my One M8, I used a Samsung Galaxy Nexus, 32gb internal plus 16gb expanded; and from 2010 to 2012, before my beloved HTC Droid Incredible - had 8gb internal maybe, plus 16gb expanded.
For me, 16gb internal seems to be more than sufficient. My Moto X Pure has 64gb adopted. I chose a Samsung Pro+ microSDXC card; that card seems to be the most consistently highly rated with regard to write and read speeds and Samsung memory has a good reputation for durability.
Now, I'm not a heavy app user, but use my device often and I keep devices two years (not on an upgrade schedule, it's just how it's worked out). My HTC One M8 still functions, and when there's a Marshmallow update available for it, I'll install it for kicks, but it'll be my backup device. The One M8 was snappy, the X Pure, even before adopting memory, seemed ever-so-slightly less snappy, but almost imperceptibility so.
My primary concern with relying on adopted storage is longevity. My microSDXC will be more busy as adopted storage as opposed to expanded, but I backup regularly, so if the storage fails before the device, I can swap out and reinstall my apps. Spotify Premium uses - by a large margin - the most memory. My One M8 had 20gbs dedicated to Spotify Premium because I had a lot of music downloaded for off-line listening. So far, my X Pure has 7.6gb allocated to Spotify. I'm sure that will go up, but it's on the adopted portion of my internal memory so has room to grow.
Of the 16gb on board, my device made usable just over 10gb. Right now, it has about 4.5gb availy, but I can clear the cache of all or certain apps (gmail, I'm looking at you!) and make more room. My primary apps are gmail, Drive, Spotify Premium, Chrome, camera, gallery and youtube. I use Google Cast also for an audio Chromecast and a first-gen video Chromecast.
All my apps work as well or better than they did on my One M8. The improved performance could be due to a number of factors: I'm running 6.0, have a better processor, my One M8 has been beat up pretty badly, etc. I just know that at this early point, for me, a 16gb with a top of the line microSDXC card as adopted storage is working great. I was about to buy a Nexus 6P and would have had to buy the 64gb version if not the 128gb. For $200 less, I'm really happy with my choice.
Notes: I'm on VZW. I activated VoLTE (VZW calls it HD Calling or something) on my account for free, then the option appeared on my device and I turned it on. Coming from the One M8, I'm amazed. My phone is actually a phone! Who woulda guessed? Also, the X Pure's camera is laughably superior to the One M8's. I used to carry my DSLR just to snap casual pics because the One M8's camera was, unless used under perfect lighting for macro shots or outdoors in perfect lighting with jacked-up HDR (which was obvious in pics), virtually useless. I love being able to take photos with my phone.
Lastly, I don't use Facebook, the app or the website and given how much battery and memory FB uses, YMMV.