Hi there 
The WD Purple drive is designed to work in DVRs and should work perfectly fine in your system. It has features that enable it to record video without interruption and thus not losing a single frame from the recording due to other processes. It has optimized sequential speeds and gives priority to the writing processes over the others. It’s also designed and optimized for video streaming so you shouldn’t have issues with that either as well as that they suppor some additional ATA commands that are specific for Audio/Video/DVR systems.
The WD AV drives can also do the job and are meant more for streaming rather than recording. As older models they don’t have all the features that WD Purple drives have and are more targeted towards extremely cool and quiet media streaming and power saving whereas the WD Purple drives should be able to sustain more demanding workloads when it comes to recording videos.
In your case both drives should work just fine so it’s up to you to decide which one you’d go with.
Let me know if you have additional questions!
Captain_WD.
