Some of my company's clients have recently started requesting the cheap DELL RAID IDE systems. Bearing in mind that most companies don't fill up their server disks before they go obsolete, the fact that there are many U160 systems out there with a single 32Gb 7200rpm HDD is plainly ridiculous. This is because even the latest 10K SCSI disks can't supply the data fast enough to the SCSI bus to fill up that 160MB/sec capacity. Neither can, to be honest, 2 SCSI disks. Now if you're looking at 4 disks, well that starts to make sense.
I know that some of my engineers (who are now well and truly fed up of me butting my head in on a regular basis, now I'm an 'expert' after having spent time on Anandtech... The boss has outgeeked the geeks, bwahahaha. ) have also played with ATA133 RAID, and the results they're getting is that dollar-for-dollar, the IDE RAID 1 option is pretty untouchable, while providing exactly what most people are looking for in a 2-drive setup - especially since there's no evidence to suggest that high quality EIDE drives are significantly less reliable than their SCSI counterparts.
Beyond this, the economics of IDE are outweighed by the advantages that SCSI provides in flexibility from a business standpoint.
I'd say that on a 2-drive RAID system, you're best off going IDE. More? Look to SCSI.