As I have said before, there is a place for both and interestingly, many members of e-mail groups have joined here (and I'm sure a few have also signed up on email groups); therefore, I believe people are happy to adapt and use both, rather than feeling the need to conform to a single style.
A forum lives or dies based on the quality of the community and the members - therefore, in my opinion format is second to content.
Moderation is another consideration. Some forums/discussion groups are actively moderated, others are much more 'relaxed' (for want of a more concise word).
Again, moderation creates the tone of the discussion and I believe people will seek out a community that works for them - format will again take a back seat to preference.
My personal choice is to a threaded online forum, purely because I believe it gives me greater control to pick and choose what I read and to sidestep the topics that are of no interest to me (all the while avoiding spurts of e-mail).
