What's the difference between [V-ながら][V2] and [V-ている]間[V2] ?
For example, is there any difference in nuance between these 2 sentences:
means that Mr Tanaka primarily does A. Incidentally, he also does B.
The main action is drinking, it's the whole context. Incidentally, it's also the unrelated opportunity to practice conversation.
Some grammar books would tell you that "ながら" is similar to "のに", to show that it's linking two different actions, and does not concern time (even though they are simultaneous):
is almost "even though I'm watching the telly, I'm concentrating on my homework." It's a kind of opposition showing that the two actions are not logically connected.
Your example sentences sound pretty much the same. My preference is for the ～ながら version, since both actions share a subject and are volitional.
The main difference, I think, between these two forms shows up when we add a second subject to the mix:
The two actions in a ～ながら construction must have the same subject, so you can't use ～ながら to construct a sentence of the form, "Person #1 did A while Person #2 did B." ～ている間, on the other hand, merely means "While [action] is happening…" or "While [condition] is true…", so you can have different subjects:
I'm going to avoid the discussion of which is the primary action and which is the secondary; those kinds of rules tend to break down in everyday usage and don't contribute much to learning.
This is the impression I get from the sentences:
I've answered your examples rather than your actual question, I'm afraid. ^^; Perhaps someone else will be able to tackle it better?