No comparison. FF is turn based, NWN is realtime.
FF targets a wider audience, anyone can pickup and play.
The gameplay is simple and story is good.
NWN is in a class of it's own. But a certain higher amount of interest in roleplaying required; it's hardcore d&d.
It has deep gameplay and a level of character customization unmatched by any game (except mmo's). You can make damn kind of character you please.
And storyline is good here as well.
I stopped playing after the first chapter (prefer simpler rpg's like Kotor), but it's damn good fun with a sorceress - you have access to hundreds of spells of mass destruction.