Well, why not try both? They are both good games, and I do howl for Planescape which is one of the coolest games in the series IMHO...
Planescape might take some time to get accostumed to but is worth the time!
IWD is a good relaxation from the quest and search driven gameplay, more or less an old fashion dungeon cleansing, but you get to make the entire party there!
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Yawning lazywolf dreaming about nice little fairies...zzzzz