I can offer a preliminary assessment of the Mini86. I purchased one a couple of weeks ago and, once initial setup was done, it has performed as advertised. It streams FLAC files from my NAS over my wireless network without a hiccup and any intervention on my part - it always works just fine. The Spotify plugin also performs as well as Spotify can. The case is slightly warm to the touch but never hot. I have the 2GB RAM version. I considered purchasing a NUC - I think that even the sub i3 Celeron processor version would work great - but was concerned about fan noise (I’ve never actually been in a room with a NUC) and local Craigslist didn’t have any used ones for sale at a price that I liked.
I think the 4GB version might be quicker at retrieving album art (a good thing) and might also be better if you’re playing higher resolution files. However, those assumptions are entirely speculative. I’ve never opened the case but assume that it contains standard DDR3 memory and it would be cheap and easy to replace the 2GB with 4GB. I also assume (again without evidence) that the device can’t use more than 4GB. [Note: I was wrong, it will take 8GB] Unless you really want to store files on the device, I wouldn’t pay for a 1TB hard drive of unknown provenance. Don’t know anything about what it might take to increase the size of the SSD.
I’m alternately delighted and appalled by Volumio. My enthusiasm for the Mini86 led me to set up two Pi 3 based players. One is great, the other looks like it may turn out to be mostly a painful educational experience.