I want to know what s the difference between SCSI Virtual Drive and IDE virtual drive?
I believe the answer to the "preference between the two" question is something along these lines.
Virtual drives in these programs used to just appear as SCSI drives. When DRM for games started checking for the CD/DVD being in an IDE drive instead of a SCSI drive, IDE options started appearing.
So, unless you're trying to play games from backup images where the DRM actually cares about the drive type, there's probably not much difference for you.
If you're just watching DVDs using it, then you should be fine no matter which one you choose.