The Auto Scan runs only if the system resources fall below a threshold level which is also unknown while Auto Scan run automatically and takes days to complete, thus complicating the design of the product because of several reasons such as the updating of the SmartScan Database which is also associated with other Scan tasks, random scanning because of newly added files to the location which is already scanned. It will scan the whole Harddisk drive.
While Scheduled scan is user dependent and the user can always run according to its routine and configure it as he likes even the scan targets can be set.
Actually AutoScan is the replacement of continuous Background Scanning and not of the Schedule Scan.
I'll add more points and all users are invited to debate on the subject.