[Xandax]

Actually - in my personal experience - SoU is much harder then the original NwN, both in regards of combat and "plot".
In the original NwN you would level much faster then you do in SoU - I found I died much more often in SoU then in NwN.

However - SoU is more closed in areas, so the feel can be more claustofobic, and there are fewer areas. So in that respect - one might be dissapointed. But then again - it is also just an "expansinon" (well - it is more an addition, because it dosen't follow the story line of NwN. The storyline of NwN is somwhat joined with SoU in HotU).

But SoU was also made to introduce new feats, classes, skills, spells and henchmen AI (amongst somethings) and is also directed somewhat at the multiplayer aspect.
SoU also offers more possibilities for roleplay, as Stilgar mentions, and there are a few twists and turns that the player can do depending on his skills and class.

However, HotU is a better single player experience though, then both SoU and the original NwN. But the HotU module was also made speficially for single player.

If I should rate them, it would be NwN < SoU < HotU.