I'm also a beginner and probably not the most qualified to answer this, but I know that much Extended and Nez offer different features.
For example with Extended you can easily load Tiled maps, Spritesheets, play sprite animations, create Camera and Particles and a lot more which in my opinion extremely useful and must have in any games.
Nez offer a simple way to create and switch between levels while Nez take care of all content management for you, also offer A* Pathfinding, Lights and more.
Both library offer very useful features, wondering if is there any limitation at all to use both in the same project.
