I think the v4v angle makes the most sense, because:

This is the niche user group you're mostly getting already (I guess).

It's far more likely than most topics to get some activity; some of those podcasts have quite a big listener base and them having a natural place to gravitate to discuss in forum-mode, you could easily imagine one or two of them blowing up into pretty big discussions. (Of course this is more likely if those podcasts talk about stacker news, which they would be even more likely to if you do this).