First just keep in mind, that the 2D rendering mode is just 3D with orthogonal projection. There is no “real” 2D. Every concept is still 3D.
Judging from the feature list, I would say they are on par. The same goes for cross platform. Both support the same targets.
The main difference is, that cocos2d-x’s main language is C++ and libGDX’s is Java (some C and C++ components for performance dependent code).
If you want to measure real world performance, you would need to run some standard benchmarks on both engines.
First tests suggest, that libGDX is much faster:
Both support the basics of what you need, so I cannot see a clear winner. It’s just personal preference. Do you prefer C++ to Java? Does the libGDX API fit you better?
There are also editors for particles, tilemaps and other stuff, which are engine agnostic or supported by both.
What both engines share is, that they are no multi-threaded engines. They leave a lot of performance potential behind. Neither they are component-based, nor they use modern features like lightweight threads. This might change in the future for cocos2d-x. A multi-threaded engine with a component based approach is kinda on the list.
In the future, performance could increase dramatically by implementing the mentioned features and also switching from OpenGL(ES) to Metal or Vulkan.