Approaches to Customization described the approaches to widget development in order of increasing complexity. Endeavor to keep the complexity of any new widget as low as possible by selecting the simplest viable approach. It is always possible to change to a more complex approach later if necessary, but it is much harder to simplify a widget after first committing to a complex approach.
Pay particular attention to widgets that will be used very widely. Simplicity and efficiency are very important in this case. A complex widget that will be used on many pages by many concurrent users can be difficult to develop without much prior experience.