I hate the dredge up this old flame-filled debate, but here goes.

What I mean by multi-tasking is this: on an IDE cable there never can be more than one process occurring.  All tasks sent down the cable must wait for the previous task to completely finish.  If a task is going to take a long time, tough luck.  On a SCSI bus more than one task can exist, and they can execute out of order.  For example, if task A is sent to a slow CD-ROM to get a chunk of data, and immediately afterword task B is sent to a speedy Seagate Cheetah hard drive, the SCSI controller can put a hold on task A and let task B go through.  This lets the main CPU spend less time waiting for things to get done and more time processing information.  I need to give a bunch of credit to Gary Field's SCSI FAQ for this last paragraph.

I was looking through Adaptec's site and I came across an article comparing SCSI and Ultra DMA.  I didn't read it over that carefully but I assume it's not totally biased.  Here's the link.