Here are the main differences between the two WD drives:
WD30EFRX (Red)
Load/unload cycles: 600,000
Warranty: 3 years
Avg. Idle Power Consumption: 2.7W
Max. Operating Temperature: 65C
Avg. Noise during Seek: 24dBA
Typical Use: NAS Systems
WD30EZRZ (Blue)
Load/unload cycles: 300,000
Warranty: 2 years
Avg. Idele Power Consumption: 3.0W
Max. Operating Temperature: 60C
Avg. Noise during Seek: 27dBA
Typical Use: General Storage