Both ways are common and accepted. In this area here, the address nodes were imported from an official database and at the time of the import, the building outlines had not been drawn yet. 

In some countries addresses are actually points and they are not connected to buildings, so in those countries it makes sense to always use separate nodes. 

In the US, if the building only has one address, you can absolutely put the address on the building outline.

For multiple addresses in the same building (also multiple units), you have to use the separate nodes.
Also for very large buildings you can put the address node into a specific place in the building outline to indicate which part of the building is the "front" part.

