What's the difference of their emphasis?
Often I felt these two are very similar.
In terms of quality, A is better than B.
is similar to:
As far as quality is concerned, A is better than B.
The phrases have the same meaning.
In terms of something means "in relation to something" or "concerning something."
As far as something is concerned means "having to do with something; pertaining to something; as for something."
If you are wondering which to use, I would try them both out and go with whichever one sounds better with the particular something being discussed.
Also, if it is already clear what is being discussed (in your example quality) from the content of preceding sentences, I'd delete the introductory phrase as unnecessarily wordy, and just say: "A is better than B."
The phrase "in terms of" cannot take a noun which does not have terms related to it. What you are literally saying is that "when using those terms related to X, Y is better than Z".
The following pairs illustrate the difference:
The basic idea is that "In terms of X" has to have terms that you use, some particular phrases, associated with X. "As far as X is concerned" just means that you can concern yourself with X. Just because you can concern youself with something doesn't mean you have terms for it, but if you have terms for something, you can definitely use those terms to concern yourself with it. So "As far as X is concerned" takes a strict superset of X's that fit into "In terms of X". "In terms of..." generally takes those nouns which describe vague qualities or general classes, which have specific terms you use when talking about them.