TLDR : WD42PURZ is superior - higher recording density, higher cache and better performance
Both WD40PURZ and WD42PURZ are CMR drives with 1 M Hour MTBF, 180TB/year workload and 3-year limited warranty. However WD42PURZ is ligher in weight which means it has less number of platters ( mostly its 2 platters compared to 3 platters of WD40PURZ). WD42PURZ also has a higher cache of 256MB and a higher transfer rate of 175MB/s which is 16% higher than WD40PURZ.
In Summary - WD42PURZ is a superior drive and will mostly replace the WD40PURZ. For sometime we still have the option to order WD40PURZ but it will slowly be replaced