i think, if you can have good books in cocos2D iPhone, and if there is none in cocos 2D X, you should start with cocos2D iPhone and switch to cocos2D X when your are comfortable with cocos2D iPhone. probably buy a C*+ book too must be useful.
i have already made a cocos 2D iphone ~~> cocos2D X, it take me something like 1 week for most part of the game, the only thing that give me some “work” was inapp paiement and Facebook/twitter integration.
porting cocos 2D iphone to cocos2D X is mostly something like :
sprite.scale = 10;
it’s greatly easy for someone know a little of C**, that clearly don’t need a “huge” level of C*+.