"Better" is subjective. One may be a better fit for a specific feature and a worse fit for a different feature.
Both are fully-functional. Both can be used to make games. Both are moderately mature libraries.
Is there some particular facet you are concerned about?
I only have any experience with cocos2d-x. I use it because it is free, am not an expert, and generally have not used 3rd party game frameworks for the 2d games I've worked on in the past, so my expectations of what is normal may be off. However, one thing about it that I notice is the extent to which a sort of sprite-centric design is forced upon you. Sprites come into existence when you allocate them and will be drawn if you add them to the scene, by magic as far as you the user of the framework is concerned. If you want to create your own drawable entities like this -- I'm sure it could be done -- but this is not the expected way in which the framework is intended to be used.
I guess to be more specific I'd like to know if Marmalade offers cross-platform access to just two-dimensional accelerated drawing exposed to the user as API calls and then has sprite-like things implemented on top of these calls.
SImilarly when dealing with different sized screens -- iPhone vs. iPad 2 vs. iPad 3 vs. random Android device -- one thing that would be nice to have is good cross-platform image classes featuring functions for scaling and composing images exposed by the framework itself. cocos2d-x offers a class CCTexture2D with which you can do this sort of thing but it is basically an abstraction of an OpenGL texture as far as I can tell, not really a generic image, so is limited...