I don't know if call it canonical. But certainly it is the version the community is maintaining. It's a fork of the mtaborda repo and converted to tonel format. I don't know how it was migrated from Sthub, but it was before any tool for migrating the history existed.
I think it must contain all the functionality available in Sthub.
Regarding Units package we never made a thorough comparison so I cannot tell the differences. But this package is intended to be usable for things beyond standard units. We're using it for financial measurements besides the physical ones.
Hope it helps in clarifying a bit the situation.