NwN will probably, mostly post at the vault on the forums there, NwN2 will probably split between there and the nexus (there is also a NwN nexus but its not used as heavily as the vault). I am registered on both. One thing with the nexus is that once registered, you can access all of the nexus games sites (e.g. Fallout, Elder Scrolls, etc.).
TR
Speaking from the nwn2 side, we used the Nexus more than the nwn1 side did for posting projects (the nwn2 community generally crossposts content to both the Vault and the Nexus), but for forum posting it's been almost all in the bioforums. My personal preference is for the vault.
On the other hand I do have about as many downloads of my modules at the Nexus as I do at the Vault, and the playerbases do seem to be generally different, so posting content to the Nexus may net nwn1 people additional players of modules and downloads for cosmetic haks (facelifts etc).