WD RED hard drives are specifically designed for NAS systems. WD Purple hard drives are a special series of hard drives recommended for video recording in surveillance systems. Seagate also has NAS recommended hard drives called Iron Wolf. In addition, both vendors also offer more expensive hard drives that are mainly used in corporate environments. It is by no means a matter of taste which hard disk you use in a NAS.

However, what many people don't know is that Seagate also offers a small 2.5-inch hard drive of up to 5TB, which can also be used in the NAS. It is usually sold separately under the name Barracuda, but you can also take these 2.5-inch hard drives out of the external USB 5TB hard drives, which have a SATA interface and can then be installed in the NAS. Since they are of course not primarily recommended for this purpose, they should always be used in a RAID.

However, I would not recommend the DS220+, because it is a 2-drive NAS. It would be better to use a DS420+, which offers 4 drive bays and is also more flexible for expansions, i.e. you can also set up RAID5 with hard drive hardware failure protection or an additional hotspare hard drive.

Whether you take hard drives from the Seagate or WD brand is a matter of taste, just like with car brands.

WD Red Plus = optimised for NAS systems
WD Purple = optimised for video surveillance recording systems