Functionality first, but I'd say that you should have a healthy skepticism on third-party libraries - not a "we'll write our own instead" but a "do I really need this feature/tracking".
In many cases the time and code required to incorporate some aspect of a 3rd party lib to do something is similar to the amount of time and code in building what you need. In that case, the dependency only serves to make your code more complex (and slow).
This is not always the case, but surprisingly often.
