1. 1. Flexibility of Data Model: MongoDB Compass provides a flexible data model which allows for the storing of unstructured data, while Postgres.js follows a structured data model that requires predefined schemas and tables. This means that MongoDB Compass allows for more flexibility in schema design and data modeling, making it suitable for complex or evolving data structures.
2. 2. Querying Language: MongoDB Compass uses MongoDB's query language, which is based on JSON-like syntax, allowing for rich and expressive queries. On the other hand, Postgres.js uses SQL (Structured Query Language) for querying data, which is a widely-used standard for relational databases. This difference in querying languages can affect the ease of use and the specific functionalities available for querying data.
3. 3. Scalability and Performance: MongoDB Compass is designed to be highly scalable and performant, particularly for write-intensive workloads, thanks to its distributed architecture and built-in sharding capabilities. Postgres.js, on the other hand, is not inherently designed for high scalability and performance, but can be optimized for certain use cases through techniques such as indexing and query optimization.
4. 4. Data Integrity and Transactions: Postgres.js provides advanced features for ensuring data integrity, including support for transactions and enforcing referential integrity constraints. MongoDB Compass, on the other hand, does not support full ACID transactions and does not enforce relationships between documents. This difference in data integrity capabilities can be an important factor when choosing between the two databases for applications that require strong consistency and data integrity.
5. 5. Data Storage: MongoDB Compass stores data in a document-oriented format, where related data is stored together in flexible, JSON-like documents. In contrast, Postgres.js stores data in a tabular format, with data being organized into rows and columns in predefined tables. This difference in data storage format can have implications for the type of data that can be stored efficiently and the ease of schema design.
6. 6. Ecosystem and Community Support: Both MongoDB Compass and Postgres.js have active and vibrant communities, offering a rich ecosystem of tools, libraries, and frameworks. However, MongoDB Compass benefits from a broader NoSQL community with extensive documentation, online resources, and community support, while Postgres.js benefits from being part of the broader PostgreSQL ecosystem, which has a long history and a robust set of features and extensions.

In summary, MongoDB Compass offers a flexible data model, rich querying language, and high scalability, but sacrifices some data integrity features. Postgres.js, on the other hand, follows a structured data model, provides advanced data integrity capabilities, and is part of a mature relational database ecosystem. The choice between the two depends on the specific requirements of the application and the trade-offs between flexibility, data integrity, and community support.