Another thing to consider is if you plan to add additional disks later. Depending on your setup you usually use one harddrive for mirroring/parity, so e.g. with 4 drives you only „loose“ 25% of space. With Synology it’s easier to add drives later (they have a mode called SHR that allows that) - with many other vendors you have to copy/restore all your data when extending with additional drives.