Hi zds, yes I think it's just the interface speed. Probably some other minor details in the control chips, circuit board etc have changed too but I think they are fundamentally very similar.
I have a 2TB EARX, it's in a Scythe Quiet Drive, sitting on foam outside my case, like the Samsung F3 1TB I had before it.
I used to be aware of the noise of the F3 certainly seeks and occationally the idle noise. I'm not aware of the WD making any noise at all!
However my PC isn't as quiet as it was and the WD is storage drive with SSD as system drive where the F3 was the system and only drive.
I've got the WD hooked up to the secondary SATA 3 controller in my system, Marvel one, and I'm quite happy with it. Performance seems fine, think over 100MB/sec potential and no noise I've noticed.