I believe that the argument here is because people don't understand the tradeoffs between the WD disks and a 15k rpm scsi drive.
The WD works very well for most of the tasks that you'd be interested in; the big buffer makes up for its relatively slow access time, and it has very high sequential transfer speeds.
However, the 15k scsi will *cream* the WD on any heavy-I/O intensive tasks. 15000rpm versus 7200rpm means that the scsi can handle many, many more i/o's per second than the WD can dream about. Think *3.4ms* access time versus *8.9ms* access time. The 15k disk adds an average 2ms rotational latency to that figure, while the WD adds an average 4.2ms. If the random i/o's are from a small enough working set that they mostly fit into the WD's 8mb cache (versus the 4mb on the scsi drive), you won't see it hit as hard, but if you're looking at a fileserver disk with many random accesses from all over the disk, access time is all that matters.
The WD will do very well, if not better, in most home user situations. The SCSI will be *much* better, however, in any any situations involving intense "disk thrashing" tasks, like file serving, video editing, or working with lots of small files.
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