Note that ssd drives make the system much more responsive (and quiet!) Synology will tell you they're not supported, but you can override this in setup.
The only gotcha with the 923+ is if you need to transcode with plex/jellyfin, the CPU won’t be up to the task. Otherwise mine has served me very well (including a 32gb third party RAM upgrade) and I direct play all my media from it.
If you do stuff like make your NAS available over the internet and use a phone app for it, then live transcoding on the NAS basically makes it a youtube-like media server for your library. It can adapt to bandwidth, etc.
If it's only going to be used at home with capable clients you don't need it
Transcoding happens on the server when the client is unable to play the content natively. For example, if the content is encoded as AV1 and AV1 is not supported on the client then the content will be transcoded to H.264 before being sent to the client.
In addition: transcoding is done if the requested quality is lower than the source quality. This mostly comes into play when accessing your server remotely on a less than optimal connection, especially on a small device where it matters less.
You really can't go can wrong. You'll get the drive, plug it in, set it up and then just use it for, well, a long time.
When I looked at all the different models, it was like splitting hairs between them. But the reality is, the model I have (DS918+) works and works and works.
The SSDs, if you get Synology, can be used as a volume, but they're REALLY pricey. Not sure how useful having a big cache would be for my uses, but is tempting.
I just got a 923+ from someone on FB MarketPlace. I've got some "old" Synology 2-bays, one with 512 MB RAM and another with 1GB. I never knew this until a day or two ago when I was comparing it to the 923+'s FOUR GIG expandable to 32 GB for very cheap (<$200, I believe). That'll be fantastic.
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If it's only going to be used at home with capable clients you don't need it
When I looked at all the different models, it was like splitting hairs between them. But the reality is, the model I have (DS918+) works and works and works.