Your first error is using module Adminstration. Nobody should it since it is crap.
Having said that "What would be better/faster?":
The second. Simply better because there will never be a performance issue, since the is no retrieve involved.
The first will do a database-retrieve if the 'User'-object is any other specialization then an 'Account'-object, and will return empty.
As we know that Current user is stored in memory
so we we call from memory in microflow or sub microflow it is same in speed and for better it should be use in sub microflow as we know that
Sub-microflows represent individual parts of the whole microflow. and for microflows
when you have very large and complex microflows, using sub-microflows can help you better manage your logic and business processes.
Hope this will help you