1. Architecture: Mongoose is an object data modeling (ODM) library for MongoDB, while Sequelize is an object-relational mapping (ORM) library for SQL databases. This means that Mongoose is specifically designed to work with MongoDB, while Sequelize can work with various SQL databases such as MySQL, PostgreSQL, and SQLite.
2. Schema Definition: In Mongoose, you define the data schema using a Schema object which allows you to specify the fields, types, validations, and defaults of the document. In Sequelize, you define the schema using models which are defined as classes with properties representing the fields and their respective types, validations, and defaults.
3. Database Queries: Mongoose provides a more expressive and intuitive interface for querying MongoDB by using chained methods and query builders. Sequelize, on the other hand, follows a more SQL-like syntax for querying SQL databases with support for raw queries and more complex join operations.
4. Associations: Sequelize provides a wider range of association types such as one-to-one, one-to-many, and many-to-many relationships between models. Mongoose also supports these relationships but with a more flexible "populate" method that allows you to reference and populate related documents in a more dynamic and nested way.
5. Hooks and Middleware: Mongoose offers a powerful middleware system that allows you to define pre and post hooks for various document actions, such as saving, updating, and removing. Sequelize also provides hooks but in a more limited way, mainly focused on specific model actions.
6. Transactions and Concurrency: Sequelize supports transactions out of the box, allowing you to perform multiple database operations as a single atomic unit. This can be useful for complex workflows that require multiple queries or updates to be executed consistently. Mongoose does not have built-in support for transactions, although it does provide some atomic operations for single document updates.

In summary, Mongoose is tailored for MongoDB, offering a more expressive querying interface and flexible associations, while Sequelize is a versatile ORM that supports multiple SQL databases and provides transaction support.