I find the Sparrow Framework API to be much cleaner and more consistent than Cocos2D's. You'll also get much better official documentation to get started with Sparrow development, and the developer is very responsive on the forum. Cocos2D's Wiki is quite incomplete and partially out of date, and the forum is so crowded and chatty that it's hard to find the buried gems.
However, Cocos2D is extremely popular and there are way more tutorials available online (check out http://www.raywenderlich.com) and books (like mine: http://www.learn-cocos2d.com/store/book-learn-cocos2d/). You'll also find more developers to ask for help.
Then there's Kobold2D which (but I'm biased) is an exciting alternative to Cocos2D with 15+ example projects, important source code integrated (cocos3d, cocos2d extensions, Lua), the entire API references available online, offline and as Xcode Help, and better cross-platform (iOS & Mac OS) support.