What is the best source to restore a backup?

Are you saying that you have a backup you wish to use, it would be whatever you have. As far as setting it up as new and getting things on the iPad, it would have to be where you have the content you wish to use. Apps would have to be downloaded from the App Store, but if you wish the iPad to resemble the iPhone, then a current backup of the iPhone would be the best to do.

An iCloud backup is essentially equivalent to an encrypted iTunes (or finder) backup. But an unencrypted iTunes (or finder) backup does not include data included in an iCloud or an encrypted iTunes (or finder) backup.

So if restoring a device from iCloud or an encrypted iTunes (or finder) backup, it’s moot.