It is certainly true that the fastest drives available at any given time are always SCSI drives, but by no means true that a given SCSI drive will be faster than its IDE competitors. By the way, we quote availability, capacity and performance for SCSI drives, but not reliability. It's probably reasonable to regard all of the drives below as equivalent to a AAA drive on the IDE pages. No practical limit on the size of the drive A single SCSI bus can have up to seven drives at once. The fastest drive technologies always appear in SCSI models first - 7200 RPM IDE drives are only just starting to appear, while most SCSI drives have long been 7200 RPM, with 10,000 and even 12,000 RPM units available.
SCSI drives tend to be built for very hard work, and last a long time.
Notice that the standard manufacturers warranty for most SCSI drives is five years instead of three. There is little point in having a faster interface unless the drive's internal data rate can take advantage of it. This is expensive. Extra complication and a greater need for fine-tuning to get the best performance. The initial cost of a good SCSI drive system is so high that you need to keep it for many years in order to justify the investment.