I am writing a simple game that animates a ball image quickly across a background image.
Would Core Animation/Quartz be sufficient for this scenario?
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OpenGL ES isn't that hard, there are lot of really good resources in the web and step by step tutorials. Can be hard at first, but once you get the grip of It things will be easy. 71 Squared has very detailed tutorials:
http://www.71squared.co.uk/iphone-tutorials/
About the question, Core Animation Is great for most apps, but in a videogame It's fundamental to draw your game efficiently and have as much control as possible.
Cocos2d Is great and I recommend It If you need to write a good game, but if you're doing It for educational purposes I recommend OpenGL, You will learn a lot with It and gain a lot of experience.