I'm torn between those two models, and I'm mostly concerned about noise and reliability. I will be using two of them in a mirrored RAID (actually, Synology Hybrid RAID).
From the latest Backblaze statistics, two WD Ultrastar HC550 16 TB models (WUH721816ALE6L0 and WUH721816ALE6L4) had an amazing reliability, but from the specs they are quite noisy: 2/3.6 dB. My issue is, that a lot of people say that they indeed make strange loud noises, but the same amount of people say that they are quiet.
OTOH, Seagate released the NT001 series this year (ST16000NT001, for example), were they caught up with other with the MTBF ratings, and from the specs they are quieter than the WDs: 2/2.6 dB. But they are also more expensive than the HC550s.
I will be having the Synology NAS in the living room, with BTRFS, and it will be used mostly for playing movies over WIFI, so the read speeds will be very low.
What are your experiences with these two models, concerning the noise?
Since you're planning on mirroring, there's one obvious solution: buy one of each, WD and Seagate.
That way you're not screwed if both drives of the same lot have the same design flaw or manufacturing defect and both fail at about the same time. In the past this has happened quite a bit.
Having only two drives means the Synology shouldn't be overly loud. If it's still too loud for you, search the /r/Synology subreddit. There are ways of quieting a box. Here are two that come to mind:
Make sure the Synology is on a very thick, solid surface. E.g. a hardwood table. Not some cheap particle board thing. Add a thin mat underneath if necessary (e.g. like a mouse pad material).
Add some velcro-like material in the right spots.
Here are two older posts from that sub to help you get started with your search: