As an old saying goes, you are comparing apples and oranges.
I can only assume you are comparing the two techniques.
With the Insert/Select technique, you are taking data that resides on the Teradata Database server and moving it from one table to another on the same server.
With the FastExport/FastLoad technique, you are pulling data from the Teradata Database server, moving it across your network, incurring the cost of I/O to write a file or a named pipe on your load server, incurring the cost of I/O for FastLoad to read the data, and finally incurring the cost of sending the data back over your network to land in a target table on the Teradata Database server.
The first technique is always superior. If the data resides on the Teradata Database, then use SQL to manipulate the data and keep it on the Teradata Database rather than moving the data across your network twice. Only use FastExport if you absolutely need to get the data out of the Teradata Database and move it to another server.