In terms of build quality, size, silence and power the Mac Mini tops almost everything as a HTPC. It's a complete off the shelf unit and works with the Apple TV remote too. Just hook up to TV and install Plex Home Theater and your set, it will handle everything you throw at it as the CPU is powerful enough to software decode media the hardware decoders might not support so you don't need a powerful back end server, heck you could use the Mac Mini as the server and put Roku's in the other rooms that are rarely used.
$600 for Mac Mini
$399 for a Lace 2 Big Quadra USB3
RAID enclosure pre-installed with hard drives or go for the 4 Big Quadra line, don't bother with Thunderbolt links those are overkill, the Lacie units are OSX compatible and quiet.
Alternatively just buy a bunch of standalone USB3 hard drives (Seagate or Lacie USB3 will work, plus they're low noise too) and store them in a cabinet near the Mac Mini. Plex is not a file/folder system you browse it's media library by the metadata it generates from scanning your media so that the files are spread across multiple drives makes no difference at all.
Instead of the Mac you could go with an Intel NUC but they aren't as slick or self build a PC it's up to you.
Android TV devices are in the pipeline but no ETA, hopefully by the end of the year we see some new hardware and if not then likely next year, maybe CES in January will bear fruit. It's more than that though whoever starts pushing out these devices needs to make a media player specific one with all the necessary codecs licensed, like Netgear or Sony could throw out a box but it only has Dolby audio no DTS etc. It's hoped Pivos next Android box falls under this category as it is a media player specific unit with all the bells and whistles.
Their are unofficial Android devices out there and you can even sideload the Plex GoogleTV apk but most of them are in poor shape.
Minimum fuss, best outcome is the Mac mini approach in my opinion.