This is a lot like Godzilla vs. King Kong. Each of these hard drives has its strengths. The WD Caviar Black has a stronger burst speed (320.7MB/sec.) than the Barracuda XT (296.6MB/sec.). The average read speeds of the two are close, but the Seagate Barracuda XT edges ahead at 115.6MB/sec. compared to 112.4MB/sec.
But where the Caviar Black dropped the ball was during my read/write copying tests. It took 3 minutes, 11 seconds to complete the write portion and 4 minutes, 32 seconds to finish the read portion. In comparison, the Barracuda XT took 2 minutes, 2 seconds and 2 minutes, 36 seconds, respectively, to complete the same tasks. The XT was more than a full minute faster on the write portion of the test and nearly two minutes faster on the read
Surprisingly, the 6Gbit/sec. Caviar Black didn't do much better than its own 3Gbit/sec. version, which needed only 2 minutes, 25 seconds and 2 minutes, 59 seconds, respectively. Of the two, I'd probably recommend the new Caviar Black for small data grabs, like POS (point of sale) transactions that can take advantage of its faster burst speed, while reserving the older Barracuda XT for large record retrievals like graphic files and such.
While SATA 3Gbit/sec. drives from Seagate and Western Digital drives will normally perform better than the average hard disk, it looks like the 6Gbit/sec. drives, while faster than their 3Gbit/sec. counterparts, don't come near to doubling their performance.
The burst speeds are an impressive 320.7MB/sec. and 296.6MB/sec. for the 6Gbit/sec. Caviar and Barracuda. However, the 3Gbit/sec. Caviar Black actually comes close with a burst speed of 248.5MB/sec.
The Barracuda XT's write speed was also impressive at 2 minutes, 2 seconds, but the Barracuda LP came in neck-and-neck with a speed of 2 minutes and 4 seconds. (The 6Gbit/sec. Caviar Black was dead last, by a significant amount, among all of the drives.)
What do we learn from this? That while the SATA 6Gbit/sec. drives do show a performance improvement, it is not necessarily a radical one.
Because the very high-end VelociRaptor spins at 10,000 rpm while the two SATA 6Gbit/sec. drives spin at a "mere" 7,200 rpm, you might suspect that Western Digital's VelociRaptor is destined to be wildly quicker.
You'd be wrong. The XT actually has a moderately faster burst speed than the VelociRaptor (296.6MB/sec. to 261.0MB/sec.) and a moderately faster average read speed (115.6MB/sec. vs. 106.0MB/sec.). The 6Gbit/sec. Caviar Black is also quicker in burst (320.7MB/sec.) and average read (112.4MB/sec.) speeds.
The real rub here, however, is that while the 300GB VelociRaptor is less expensive than the Barracuda XT, it also has 1.7TB less capacity -- and is more expensive than the Caviar Black 6Gbit/sec. with 700GB more storage room. That's quite a lot to give up in either case.