SQL: transfer database schema
ALTER AUTHORIZATION has a more general use for changing ownership of databases, types, keys etc. There is an ENDPOINT example n this MSDN link



  ALTER AUTHORIZATION can be used to change the ownership of any entity that has an owner



ALTER SCHEMA does exactly one thing



  Transfers a securable between schemas


In this special case you've highlighted, they do the same thing, There is no difference.

I'll guess that ALTER SCHEMA was added for completeness (eg there is CREATE and DROP why not ALTER) or in preparation for other features (such as renaming a SCHEMA, say)