The differences between Vue 3.0's Composition API and Vue 2.x's Options API are as follows:

Code Organization:

Composition API: Uses the setup function to centralize the management of a component's state and logic, making the code easier to read and maintain.
Options API: Distributes component state and logic across different options such as data, methods, computed, and watch.
Logic Reuse and Composition:

Composition API: Facilitates easier extraction and reuse of component logic without needing mixins or other abstraction mechanisms.
Options API: Typically requires the use of mixins or higher-order components to reuse logic, which can lead to naming conflicts and tight coupling between components.
Type Support:

Composition API: Being function-based, it integrates more easily with TypeScript (functional programming).
Options API: May require additional type declarations and decorators.
Reactivity Declaration:

Composition API: Explicitly creates reactive state using ref and reactive.
Options API: Reactive state is typically created implicitly within the data option.
Lifecycle Hooks:

Composition API: Lifecycle hooks like onMounted and onUpdated exist as functions within the setup function.
Options API: Lifecycle hooks are defined as component options, such as mounted and updated.
Template Usage:

Composition API: All variables and methods returned by the setup function can be directly used in the template.
Options API: Data and methods in the template need to be defined separately in data, computed, methods, etc.
Dependency Tracking:

Composition API: Provides more granular dependency tracking, where only the actual state in use triggers component updates.
Options API: May cause unnecessary component re-renders in certain scenarios.
Code Splitting and On-Demand Import:

Composition API: Facilitates easier code splitting and on-demand imports since related logic can be more easily organized together.
Options API: Code splitting and on-demand imports are usually more complex and redundant.
Readability and Maintainability:

Composition API: For complex components, it is usually easier to understand and maintain due to the centralized logic.
Options API: For simple components, it may be more intuitive as the API is dispersed.
Community and Ecosystem:

Composition API: As a new feature in Vue 3, it may take time to build an ecosystem around it.
Options API: Already has a mature community and abundant resources.
Both have their advantages and disadvantages, and Vue 3 also supports mixing the two, allowing developers to choose the API that best suits their specific needs.