nc-snapshot uses no additional space, while backups with data store a whole copy of your data, which could be many gigabytes. You can have hundreds of snapshots of a 20GB datadir in a 100 GB harddrive, but only five data backups.
Yes, I recommend doing that
Due to the nature of BTRFS snapshots, the snapshots need to be created in the same drive. There is no option to change the path for simplicity.
If you want a second copy of your snapshots in another hard drive, or in another machine, you can use nc-snapshot-sync.