New Synology NAS or DX517 expansion

[Enquiries]

I am using Synology DS918+ with 4x4TB WD RED PLUS in Raid5 and I am really happy with it, but the available space is at 80% threshold. I simply need more space. What would you recommend? A new DS920/DS923 class Synology NAS or DX517 expansion of the existing one? What are the pros and cons of both approaches? This time I would need 12/14/16TB disks and I would rather get best value rather than most capacity or cheapest option.