For what’s it’s worth, I’d say that you should post up your figures in which ever forum you prefer. The Figure forum puts the emphasis on the figure work and painting, while the AFV Figure forum rather links the figures to vehicles, dioramas and vignettes.
The AFV Figure forum probably gets more attention because it’s a sub-forum of the AFV forum (which, I believe is the most frequented of the KM Forums). On the other hand, the Figure forum is probably frequented by folks who concentrate on the skills and craft around that modeling genre. I suppose it might depend on what sort of discussions and feedback you’re looking to generate.
However, there are no hard and fast rules about which forum to use, really. I don’t think anyone is going to tell you that you’ve posted your figures in the “wrong” forum.