Ben Armstrong writes about the differences between virtual SCSI and virtual IDE disks in the Microsoft Virtual Server environment.  Read it here:

As you may or may not have known, there are limitations to using virtual IDE and virtual SCSI disks, much like their physical counterparts.  What you may not have known is that there is a performance difference as well.  Ben discusses it, but doesn't go into specific numbers. 

It is also important to note, that creating a brand new SCSI disk and then installing an operating system on it will seem much slower than when using IDE.  Microsoft released a virtual floppy disk that contains the SCSI Shunt Driver needed to properly emulate the SCSI controller during the OS installation.  During the install, hit F6 and then use the virtual floppy to load the emulated SCSI driver.  This should speed things up when installing.

From Microsoft:

Virtual floppy disk for pre-loading emulated SCSI drivers