SCSI vs IDE

[mark]

Is there much of a difference in speed between using a 50pin scsi drive (or indeed a with an adapter 68/80pin drives)  vs IDE drives on the internal bus of a Pre G3 - PCI Power Mac. Or is it mainly a cost per megabyte issue in that the larger IDE drives are a lot cheaper. At what point (which version of SCSI) does SCSI become faster than IDE say ATA66/100 with a 7200 drive?