Qt 5+ integration with a graphics engine

[immuner]

Hello,

i have a graphics engine written in C++ and OpenGL 3.3+ and exposed in a DLL. The window interface is currently based on FreeGLUT. I want to replace this basic interface with Qt 5+ so that various engine tasks can be controlled by Qt widgets. Since this is going to be part of a larger project, i would like to make the right decisions from the start. I have read (as much as i can) some of the Qt documentation, but i would like some advice as to which way is the right one to go.
The basic goal is to maintain the performance at the highest possible as the engine is used for high performance visualization tasks, while having a rich UI (using styling, etc). I do not want to use Qt to replace any part of the engine's rendering, simply for UI.
So if i have understood correctly i can either:

• Use QMainWindow as a basis which provides me with toolbars, etc, add all the qwidgets i need and embed the engine in a QGraphicsView (i have already done that).
• Use QWindow as a basis (this is new in Qt 5) and embed both the engine and the GUI elements there (not sure how to do the latter)

What would you suggest? Is there an alternative way for this?

Thanks in advance