The entire WD TV Live family offers HD playback and is currently the most up-to-date product family when compared to the older WD TV’s that do not have Live features over the network (Such as internet services like YouTube and the ability to play content directly from your computer without using an external drive).
The non-Live WD TV’s are older and they only play media from external devices connected directly to them, so they should be less expensive. I could suggest to verify if the store you have in mind has a price mismatch or incorrect/partial product names to prevent obtaining the wrong product.
Also, here is a link to the current WD TV product line: