Where does one begin with all this good stuff? I'll just jump in and add my peanuts to the gallery.
The new ATV is a nice update to a behind the times media player. It is even nicer that Apple has seen the light and allows (if nothing changes drastically) for apps like PLEX and VLC. In theory, the newest offering should by hardware alone, be able to handle these apps well and possibly improved performance of streaming apps (Netflix etc.) based on ability to buffer better.
What this new ATV can't do (or rather wont) is offer up true HD audio and in DTS-Master. Assuming PLEX can do its own magic, at least we'll still see DTS core being played from files that have only DTS-Master included. We wont see 4k at least on this incarnation either.
So, is the ATV worth it? Yes if you have already invested in ATV and *like* what it has to offer.
For others of us, who are not so married to Apple eco system, there are adventures to be had elsewhere. I recently started to dabble with the NVidia Shield TV. While it wont do "Apple" so to speak, I get instead the Android counterpart and far more flexibility. The remote seems to be well liked but I find it too primitive or over simplistic. I consider this device to be far more bang for the buck than the ATV with all it can do and that includes 4k. Like the ATV, it doesn't presently do HD audio simply because NVidia didn't acquire the licensing. What does set it apart is that it is capable of handling DTS-Master and Dolby's counterpart if* they acquired the license (ATV wont have this). Last negative, presently out put is at 60 hrtz so 24p movies are converted. Again, we may see NVidia listen to its request board/forum (as they did with other items) and fix this for true 23.9xx/24p playback. Last, it comes with PLEX and you can install Kodi!
If you are really interested in PLEX or Kodi (XBMC), yes the Mac Mini is not a bad choice at all as it can do other things too. Unfortunately, between the EFI and OSX, there is no HD audio streams to be had (Apple's decision). If you want HD audio, your Mini can still provide it with PLEX or Kodi if you change the OS. Either Windows or Linux on the Mini, running PLEX or Kodi will work. I personally can only speak for Kodi at this time.
All said and done - strictly for my tastes and it is subjective
#1 Mac Mini for pure playback capability when using PLEX or Kodi
#2 Nvidia Shield TV for flexibility and output but no Apple eco system.
#3 ATV ideal choice for those who like the Apple eco system and don't care about HD audio and other matters.