so the NAS CPU is better for plex because it has better iGPU for transcoding. But do I need transcoding if I have good end device like LG OLED65C8 TV?
I have been told to get the Dell because it has more powerful CPU, but you are saying that is has worse iGPU so I should get the NAS right?
You will need transcoding unless all media is in the exact same format using the same codec that also happens to be natively supported by Plex. Afaik, TVs are too dumb to decode the video you sent to them so the encode/decode process is always done server side.
I also just caught a mistake on my end, most Xeon E3 chips have no iGPU. The 1225v5 actually does, so you're good there. The iGPU in that is actually a lot faster.
Forget what I said, the Dell would appear the better overall machine