1. Efficience of coding
2. Simplicity of debug: you can firstly develop your game using cocos2d-html5, and debug your game logic on Chrome which is also easier than debug with C++.
So all depend on your need, if you develop a simple game and want a quick development, then cocos2d-html5 with JSB is perfect for you, but if your game is more complexe and using many features like particle system, OpenGL effects, armature animations, etc... then may be you'd like to test the performance first.