SoZ uses the Aurora Toolset for the scripting of the dialogues. Icewind Dale 2 is far more primitive compared to this. It's like comparing a Kia to a Tesla. Also, Storm of Zehir in today's age is now considered almost ancient. So while this may prove to be true, Realms Beyond will use the UrWelt Engine for it's dialogue script, fashioned from the best of both worlds and everything in-between.
How so?
SoZ has a function other than single player called the 'Toolset' and 'DM Client' that is used by players who which to "play God," and create their own worlds (called a Persistent World). Part of a PW is the dialogue of NPCs. SoZ is the last expansion of the Neverwinter Nights series and it uses this program to create fairly advanced dialogue boxes and options. Icewind Dale 2 and IWD 2: HoW was simple in the fact that: 1) It was only single player, 2) The dialogue was a straight forward multiple choice option. In the end, the only real difference is that in the former, sometimes you need rolls and checks of various skills to either complete a particular dialogue; for instance a persuasion check of DC 34, for example, to yield a conversation in your favor. Other times, you may need a taunt roll higher than a DC of 26 for example. This functionality is efficient and advanced but the drawback is that it is exceedingly time-consuming. If you are a world-creator, you can spend literally
months on just one conversation, let alone a complete area. So while comparison between the two systems is a nice thing, realistically speaking they would need to implement a system that is simpler and quicker to use while not negating the world-building aspects that Neverwinter Nights and NWN 2 had. I believe they are currently working on this.