If you play on a multiplayer server that has not transitioned to the NWN:EE version you will wish to remain with the original. If you do not play multiplayer? I suggest having a gander at NWN:EE. There are more updates coming relatively soon.
There's a lot of value in playing existing fan-made single player mods. During the Head Start period, you can expect the odd glitch in EE, which you can work around by loading the saved game into Diamond Edition for a while.
You can have both if you want, and then edit the nwn.ini file to point at the same folders for haks, portraits, etc. (That's what I did, so that I don't have to have two copies of everything on the hard drive. Since all those custom resources are the real serious space-eaters, it works.)
Both is a good idea unless the old version is incompatible with your OS or hardware. Most of the EE features right now seem less matured than their community counterparts so I couldn't honestly say it's a 100% upgrade in all areas. Keeping both is a safe bet so you can access all content properly.
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There's a lot of value in playing existing fan-made single player mods. During the Head Start period, you can expect the odd glitch in EE, which you can work around by loading the saved game into Diamond Edition for a while.