Ah PowerQuest isn't a scripting language, it's a set of tools to use with the Unity game engine to make adventure games.  

Both AGS and PowerQuest use a scripting language. AGS is based on C++ and PowerQuest is c#. But they're very similar really. 

Hope that makes sense.

You can do more with unity and PowerQuest, but AGS is easier to start out with, if you're worried. And the things you learn in AGS are useful if you want to change to PowerQuest later anyway.