We started writing Kim around the same time as the Marshmallow project began as we found it wasn't suitable for our needs at that time, though it has come a long way since then.
They are very similar projects and have similar functionality, but Kim has a focus on making it relatively simple to do unusual or 'advanced' things.
For example, Kim supports polymorphism out of the box, if you have an AnimalMapper subclassed by a CatMapper and a DogMapper, passing a Cat and a Dog to AnimalMapper.many.serialize() will automatically do the right thing in a similar way to SQLAlchemy polymorphism.
We also have support for complex requirements such as nesting the same object to itself (useful when your JSON representation is nested but your DB representation is flat,) serialising multiple object fields to a single JSON field (eg full_name consisting of obj.first_name and obj.last_name,) a range of security models for marshalling nested objects and a fairly extensible roles system.
In general we've followed the philosophy "Simple things should be simple. Complex things should be possible."