First of all, welcome to the forum!
I've personally grown comfortable with cocos2d v2.x and in my opinion the features of cocos2d v3.x don't exactly justify the redesigns implemented into the engine. So I haven't chosen to upgrade from v2.x to v3.x but Cocos2d v3.x is the latest version. It is also the only version that is still maintained. So if your new to cocos2d, I'd differently recommend the latest version.