I had heard that too, and I thought it was interesting that the non X drives were better. Yet as I read various reviews, they all seemed to agree that there was no real difference beyond the clear side. I just checked the Western Digital site and could find no substantive difference between the two drives, other than the clear cover on the X drive.
From Newegg, about the X:
Crystalline polycarbonate lens - Provides clarity, structural integrity, electrostatic discharge (ESD) protection, durability and chemical neutrality necessary for the highest performance and reliability.
Native Command Queuing (NCQ) - Increases data transfer in high-performance multi-processor, multi-threaded environments.
Rotary Acceleration Feed Forward (RAFF ) - Optimizes operation and performance when drives are used in vibration-prone multi-drive systems. Patent pending.
Also from Newegg, about the ADFDRTL:
Reliable - Designed and manufactured to enterprise-class standards to provide enterprise reliability in high duty cycle environments. With 1,200,000 hours MTBF, these drives have the highest available reliability rating on a high capacity drive.
RAID-specific, time-limited error recovery (TLER) - A feature unique to WD, prevents drive fallout caused by the extended hard drive error-recovery processes common to desktop drives.
Rotary Acceleration Feed Forward (RAFF) - Optimizes operation and performance when the drives are used in vibration-prone, multidrive systems such as rack-mounted servers or network storage.
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Not sure if the X really doesn't have TLER. I don't see a comparison of the two 150s on the WD site.