There is really no functional difference between MTS and M2TS files. Both are High Definition files associated with the AVCHD video format. An M2TS file extension can be renamed to an MTS file extension and vice versa.
An AVCHD video is normally copied from a camera to a computer using the drag-and-drop method has an MTS file extension. Whereas, AVCHD video imported with PlayMemories Home™ or other software is typically assigned the M2TS file extension.