SCSI versus IDE

Comparison
In some computers, SCSI is better. SCSI is a smarter bus than IDE. There are many steps in the SCSI data transfer. This complication often makes IDE a faster comparison. But, on OSes that allow multitasking, or if you often use several programs at once, the SCSI drive may be a better choice because this extra intelligence of the SCSI bus will be used.
SCSI devices can communicate independently from the CPU over the SCSI bus. This is due to the fact that each device has its own embedded controller. Data can then be transferred at high-speeds between the devices without taking any CPU power. IDE, likewise, uses controllers on each device, but they cannot operate at the same time and they do not support command queuing.

Final Thoughts
For most users, IDE is fine and offers very good performance. It is only when you get into serious multi-tasking that SCSI is a real benefit.
The fact that SCSI is just all in all faster then IDE is just a myth. One must also consider that all hard drives, IDE and SCSI, are limited not by the interface but by the components. Seek time and other factors are the biggest drawback on performance.