Comparing 5900/5400 and 7200rpm drives for NAS

I have (more or less) decided to go with the Synology DS920+ over the DS720+.
I will be bumping the RAM of the DS920+ to 8GB with an additional 4GB module on purchase.
What hard drives would you recommend for the 4-bay?
The NAS that I am upgrading from has two “Seagate Ironwolf 4TB / 64MB / 5900 RPM / ST4000VN008” hard drives (RAID1) on it, which are roughly 3-4 years old with very little use.
How is the performance of the 7200 RPM versions compared to these 5900 RPM ones? Is it noticeable when mainly using the NAS for backups and PLEX?

One idea I have is to:
Buy two more of the same model (Seagate Ironwolf 4TB)
Pop the two new drives into the new NAS
Set the new NAS up to run SHR
Do manual migration of data from the old NAS running DSM 4.3
Pop in the two old drives into the new NAS, format them and run them as part of the SHR


Having an off-site backup would be very smart idea. In case of fire/flood/burglars.
But if you do want to repurpose your old drives you could do that. It is recommended to match the same performance drive. Otherwise your weakest link will determine overall performance. Comparing 5900/5400 and 7200rpm drives would result in very similar performance if you only use your NAS for backups and Plex.
You would notice a difference if you were running virtual machines, web server or other IOPS demanding tasks. Higher RPM will often mean betters IOPS. But nothing compares with SSD.

I hope this helps.

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