Unfortunately, we're not able to point to one product and say it's superior to the other in all scenarios. The Seagate NAS 4TB performed a little better in our single user tests, but the WD Red 4TB stuck back in our multi-client test. Both products cost roughly the same too, so there isn't a significant difference for most use cases.
I'd have to say these two products are equal in almost all aspects. The single user performance is a little better with the Seagate NAS. The higher speed platters give the Seagate NAS an advantage, but the difference is minimal. When playing media on a device from a NAS, the single user tests are moot, since the media plays back at a constant rate, and it doesn't matter if you have overhead until playing more than one media file on multiple devices.
The Western Digital Red 4TB performed a bit better in our multi-client test. I think Red has better cache algorithms and that allowed Red to perform at a consistent rate, closer to what we see with enterprise HDDs.
The next big test for both Western Digital and Seagate will come in capacity. With hard drive pricing back to pre-flood levels, consumers can turn their attention to capacity size per drive and per dollar. 4TB models have become common, but media sizes are increasing. Blu-Ray ISO files are roughly 50GB and 4K content is on the horizon.
Early adapters of 4K media content will burn through a 4-drive RAID 5 array with 4TB drives quickly. This is a scheduled discussion for Storage Visions 2014 in just a few months. It'll be interesting to hear how both Western Digital and Seagate plan to tackle increased data rates and sizes of next-generation media.