Block storage is $.10/GB; Spaces is $.02/GB. I was hoping there was a glue layer that would allow PostgresQL to use Spaces, but such a thing might not exist or be performant enough to be worth building.
Spaces is for object storage, stuff like videos, images, documents, etc. You can store references(links) to this objects in postgres if you want (very common thing to do).
Block storage is for adding volumes to your droplets (like adding a second hard drive to a computer). One advantage is that they are separated from your droplet and replicated, if your droplet disappears or dies, you can reattach the volume to another droplet. So one common use is to place the data dir of PG for example, in the external volume. You should still have proper backups. DO also has a managed PG service.
