I cant see the point of spending the extra on Synology unless you definitely need it to be Plex Media Server compatible? There are just as good NAS boxes out there for less. But if you do need it for Plex, then you cant argue.
I'll be getting a NAS soon, but as i use my Mac mini for Plex (direct to TV with HDMI), it will just be a simple 2-bay chassis with a couple of 2TB discs in it. Ive outgrown my external 500gb HDD slightly.