Cocos2d-x and Cocos Creator vs Starling

25% of iPhone games are made using Cocos2d-x.

This means you will not be alone in development, and will have access to a large community. You'll know you are developing for an engine that works.

Documentation includes a programming guide, API reference, video tutorials and massive reference test code project showing all functions and giving working code to the user.

A new feature since Cocos2d-x v3.1 is support for 3D models (in your 2D game), not only this but support for skeleton animations is included too! This awesome feature allows for impressive characters in your game along with easier, more fluid and realistic animations.

It supports Lua and Javascript with full feature support.
Especially with Cocos2d-JS you can develop games cross web and native, and the native solution have great performance with JS Bindings, much better than hybrid solution.

Cocos2d-x forums are active.

Cocos2d-x is not only open source but also supported by Chukong Technologies of China and USA.

Regularly updated and adding support for the latest technologies. 2014 has already seen the release of Version 3, a new Cocos Studio development toolkit (optional) and support for new technologies like skeleton animation systems Spine and Adobe DragonBone.

Cocos2d-x is C++ based engine and it has CPU advantages for most platforms because of that. It uses polygonal mesh methods for sprite rendering for using GPU advantages. (You also use quad methods for benefit CPU).

Because it is based on Pyglet.

On top of supporting pretty much all existing platforms (except consoles), Cocos Creator (Cocos's IDE) allows you to write 1 code that runs on Android, iOS, Windows, MacOS and HTML5 (not Linux though).

Through the use of SDKBox you can easily integrate 3rd party SDKs and plugins for each version of Cocos2D (Lua, C++ or JavaScript), you just choose the SDKs to integrate and SDKBox will do the rest.

For example, if you want to add a rating plugin, you use sdkbox::PluginReview::init(); and if you want to add the Vundle Ad Network SDK, you use the one packaged in SDKBox SDKBOX sdkbox::PluginVungle::init();.

It has a built-in Python interpreter that allows for easy debugging.

Cocos Creator (Cocos' IDE) comes with scene editing, UI editor, animations & particle editors and whatnot. It's also easy to use and pretty intuitive if you read the official documentation & tutorials. Way way better than the old CocoStudio.

Hundreds of video tutorials available.

Starling is just 12k lines of code and doesn't try to do everything — but what it does, as efficient as possible.

With Feathers, you can easily add great user interfaces to your games — or even create business apps with Starling.

Starling is available for free with code available on GitHub.

Forum is always active with knowledgeable developers and with lot of inside info, and post mortems.

With UI, you can design it in Flash Professional (powerful 2D editor), export to swf file, use GAF convert swf file to gaf file, finnally, load gaf file to Starling. GAF can convert a lot of file swf to one atlas. Alway use same resource in design files (fla) to optimize atlas size, avoid duplicate asset in atlas.

You can use flash to create animation for starling.

Let the GPU do the rendering, the CPU has more important tasks to do.

It can be exported to web flash player, iOS, Android, Windows, and OS X

Using AIR's native extensions any native code that can be written can be run and used by the game engine.

New features are added regularly.

Starling can run more animated display objects than Unity2D and many others frameworks at 60 fps.