I'm debating the TS-453D-8G-US and the TS-464-4G-US. I'll add the 16 GB to the 464. I don't intend to use it for an NVR as I use a ubiquity setup for that. I mostly want to be able to host files access remotely and expand on PLEX.
Robbie's done a direct video on this topic... it's on youtube, but I can't post links - uploaded 6th August.
Bottom line - there's really not a lot between them.
Whichever you choose - get the lowest RAM you can - then buy crucial RAM and upgrade it for less money - end result - higher spec, lower cost.
These two are similar NAS. Regarding Plex there might not be a big improvement. Both are limited to 1080p transcoding remotely. But you can game simultaneous streams in FHD using new 64series model.
I hope this helps.