I would personally say Cocos2d-x because its great for simple 2d games, when going 3d UDK or Unity is the way to go.

Cocos2d-x is more hands on in terms of coding whereas Unity is less, so that will also influence your decision.

Unity is more refined but for simple games can seem overwhelming.

Ahh i see .. Well there are a couple people that say its very easy to make 2d games in Unity .. Its its good because you can make 2d games and put 3d looking features in there .. Plus it uses c# which is very nice. 

--SonicX278

3d features is indeed a cool thing.

C# is more personal preference.