At least according to Tom's Hardware, which just released their test results benchmarks of WD's Purple and Seagate Surveillance hard drives.
Overall, deciding between the Purple and the Surveillance HDD comes down to your application. With fewer than eight drives, under a known read/write workload, the WD Purple stands out. Going beyond eight drives with a write-only workload definitely favors the Seagate Surveillance HDD. In both cases, the two drives offer great surveillance performance, exceeding what you'd see from mainstream desktop drives, while selling for far less than their enterprise-class siblings.
This qualification seems silly, however, as most surveillance systems are predominantly write-intensive, since most users rarely search for video, seemingly making Seagate the best choice overall.
It's also worth nothing that these tests were performed using benchmarking tools, not with actual cameras or even real video streams, so no real world performance issues were discussed.
WD Purple for Small Systems
One other key point in these tests: According to WD, the Purple line is not intended for rack mount use, only small drive count standalone systems:
Specific to the Purple, WD suggests no more than eight in a system, and they're not intended for rack-mount applications. This is mainly due to a lack of Rotary Acceleration Feed Forward (RAFF), a technology that overcomes the effects of vibration introduced by other drives in an enclosure. In server applications, where dozens or hundreds of drives can share the same rack, RAFF is necessary. Clearly, WD doesn't believe that functionality is required for the Purple's target market.
Readers may see our original overview of WD Purple and Seagate Surveillance drives for more background.