There is no one right answer to the question of whether you should use plugins or extend components. In reality you are likely going to need both in the right circumstance. If you extend components too much, you might have a hard time refactoring or reusing bits of functionality elsewhere. If you have too many plugins, it may be hard to handle specific features of controls in a manageable way.
If you are likely to use components such as Windows, TabPanels, GridPanels, Panels (perhaps with specific layouts) repeatedly to show very specific information, then that could be a candidate for subclass.
In object-oriented terms, if what you build "is a" component you are extending, then that is a good fit for extending. If you are repeatedly composing components by including others, then you can make that composing component into a subclass of whatever it is and internalize the composition. If, on the other hand, you are augmenting functionality and want to package...