What is the benefit of a 4-bay NAS vs 2-bay ?
I will be buying either a Synology DS 720+ OR a 920+ as soon as they become available.
The overall cost difference does not appear too great, once the disc cost is added.
But if I only buy 2 discs (which can easily meet my capacity needs) are there any (future proofing ?) benefits in going for the 920+ ?
The main benefits having 4-bay NAS are price per terabyte and expand-ability. If you buy a NAS you will very likely have a RAID. Unless you have a backup in place and time of restoring backups do not bother you.
Mirror or RAID1 will keep a copy of a first drive. This means that you can only use storage capacity of a single drive. With a 4-bay you can set up RAID5 on first 3 drives or all four. This means that you loose only one drive and protect other 3 or more drives.
If you wanted 12TB capacity. With a two bay it would look like this :
2 * 12TB (one drive used for redundancy) = £274 * 2 = £548
with a 4 bay it would look like this :
4* 4TB (one drive used for redundancy)= £89 * 4 = £356
More drives you add, cheaper it gets.
You can even start with a one or two drives in a 4 bay. This allows you to upgrade to RAID5 later when you add third or fourth drive. You can also use extra bays for caching you you use it for virtualisation, webserver or multi user environments. Or as per DS918+ you get nvme cache slots built in.
I hope this helps.