As a spin-off of my question on "Advice on buying an encrypted disk" -- and in an effort to address the question "how is hardware based disk encryption better?" -- I've started a new question.
My understanding is that hardware based disk encryption is more secure because the keys are embed in the system, require physical access to get, and very specialized knowledge to extract them. Beyond that, the hardware encryption doesn’t require system resources to perform the encryption/decryption process and therefore allows for better system performance and data access speeds. That said, this is based on my understanding from years ago, and very possible this has changed, or even more likely, that my understanding is not completely right.