WED should use the engine in the viewports and become a modern WYSIWYG editor.
If I remember correctly there was some talk about this but then GameEdit came up and now the idea seems to be discarded? GameEdit is clumsy and having to use 2 separate level editors is a very strange concept which isn´t user friendly!
I use
Unity since a while. Their WYSIWYG editor is very nice. In my opinion Unity is the competitor which is most similar to Gamestudio and once they have a Windows port of their tools (which will happen sooner or later) a lot of people will move to Unity if Gamestudio hasn´t improved in the areas Unity does much better.