TLDR : The difference is in the number of platters and the cache. The WD22PURZ is newer with probably 1 disc and 256MB cache.
There are 2 variants of 2TB Purple drives that can be ordered for the Q4 22 channel. The difference between the two is the higher recording density and cache.
Both WD20PURZ and WD22PURZ are CMR drives with 180TB/year workload and 3-year limited warranty. However, the WD22PURZ is the newer model with probably only 1 platter and the WD20PURZ has 2 platters. The difference is evident in the weight - the WD22PURZ is lighter. However, since WD does not specify the number of platters in the spec sheet, it is difficult to be sure.
However, it is certain that the WD22PURZ has a higher cache of 256 MB and a higher transfer rate of 175 MB/s.
In Summary - WD22PURZ is the newer, ligher and Faster model and you should choose WD22PURZ over WD20PURZ if your NVR/DVR supports this new part number.