Well, I don't actually think it's one or the other. The two types of gathering are not necessarily as different as you suggest. As has been said time and time again on Mudcat, all depends where you are geographically and what the local scene is like.

At a time when Folk Clubs are complaining about falling attendances...

Not in my part of the South-East (Surrey, Sussex...), but perhaps in yours. There's also a great buzz in many of the informal acoustic sessions down here - just depends who turns up on the night and when and where it happens. Oh - and on the enthusiasm of the organisers as well.