VSS Full Backup vs VSS Copy Backup

Both create backup of all the files, and VSS Full Backup modify the archive bit while VSS Copy Backup leave it. So if VSS is the only backup you use, you should use VSS Full Backup since the second backup would be an incremental backup.

Imaging the situation you use VSS Full Backup with third-party backup software - the third-party backup software creates a backup on Monday, and you use VSS Full Backup to create a backup on Wednesday, then the third-party backup software creates a second backup on Friday, the second backup would only include the difference between Wednesday and Friday since VSS Full Backup clear the archive bit on Wednesday.

Hence, the backup behavior of the third-back is not expected. In summary, use VSS Copy Backup if VSS is not the only backup solution and use VSS to create full backup always and thrid-party backup in a combined (full + incremental) way.

Refer to diff of vss full backup and vss copy backup.