See it like this:
For your game, you want something function, but doesn't slows your game down to a slug. CEGUI is (appearantly, I never worked with it directly) one of the better options for that.
For your editor, you probably still don't really want it to slow down by a lot, but it's a stand alone application that makes use of your engine (doesn't even need to), The stuff your UI needs to do in here is probably a lot more varying than what it should be able to do in your game, or more data oriented I guess. So QT might be the better choice here, provided it does what you need for the editor.
So it's just weighing off your choices. What do you need, do you need full speed for the editor etc.
I didn't do any proper GUI work to give you the final solution, but if I would, those are a few of the questions I would ask myself when making something.
My game being slow is not good so thanks for the advice on using CEGUI.
For the game engine, i think i may be confused. Doesn't unity/udk game engines have user interfaces i.e. you want to edit the world then you click on the world editor button and the screen changes, you want to import/export a model then you click the import/export button and a drop down menu appears, you get the picture right. I may be confused. Can some post a photo of the first screen they see when they click on their game engine icon and it loads, it'll help me get a clearer picture.
