Huge thing you need to recognise here.
In my experience, if you intend on doing almost any transcoding, NAS boxes can be a bit lacking. Transcoding a 4k or even 1080P stream takes a decent amount of CPU grunt.
So consider things like
Are all your clients wired or do you have brilliant AC WiFi with little congestion/interference?
Will you ever want to use the Plex App on your mobile when you have 4G/A Hotel WiFI connection?
Will you ever expect the server to be doing more than just Plex (CCTV/Newsgroup Downloader/etc)
Persoanlly, I don't rate NAS boxes. I'd be using a microserver with FreeNAS/UNRaid ro something like that.
The Dell T20 with the Xeon option was ideal if you wanted something that can have the ability to do transcoding. The new model (T30) is out now with the Xeon CPU, but its not appeared yet with the great deals the T20 used to have.
I've got a *slightly* overkill option and have an ML350 G6, with Dual X5650's and 92GB RAM. I run out of devices to transcode to before I run out of CPU power.
All of this said, if you are going to do everything as direct play and never need to transcode, you can get away with a NAS if you dont want it to do much else at the same time.