The primary difference between them is the way plugging behaviors of a entity. Nebula, actually it is Managlore though, provides a mechanism which can dynamically compose behaviors of an entity at a beginning of an application. Not likely that, a programmer should statically declare entity's behaviors in a source code in Zombie.
There is another difference in communication between classes. Mangalore provides the message system which is used for communication between each of properties. It helps decoupling of properties so it makes the system to be flexible and reusable. But Zombie directly calls functions of other component inside of a component.
In Magalore, behavior of an entity is called for 'Property' though it is 'GamePlay Component' in Zombie.