1. Ease of Learning and Simplicity: Flask is known for its simplicity and minimalistic design, making it easier to learn and understand compared to Rails. Flask follows a "micro" philosophy, offering only the essential features, while Rails incorporates many conventions and abstractions, resulting in a steeper learning curve for beginners.

2. Development Speed: Rails is renowned for its emphasis on convention over configuration, providing many built-in tools and generators that speed up the development process. On the other hand, Flask gives developers more freedom and flexibility, allowing them to choose their preferred tools and libraries, potentially leading to slower initial development.

3. Size and Flexibility: Flask is a lightweight framework that allows developers to customize every aspect of their applications. It provides the freedom to select third-party libraries and components tailored to specific project requirements. In contrast, Rails is a full-stack framework equipped with numerous predefined components, making it more opinionated and less flexible compared to Flask.

4. Community and Ecosystem: Rails, being one of the oldest and most popular web application frameworks, has a large and active community. This translates into a vast and mature ecosystem of gems (libraries) and resources available to Rails developers. While Flask's community is smaller in comparison, it still provides a range of extensions and plugins, though not as extensive as Rails.

5. Database Support: Rails relies heavily on its built-in Object Relational Mapping (ORM) called "Active Record," which simplifies database management. It seamlessly integrates with popular databases such as MySQL, PostgreSQL, and SQLite. Flask, on the other hand, does not provide an ORM out-of-the-box but instead supports multiple ORMs like SQLAlchemy, allowing developers to choose the one that best fits their needs.

6. Scalability and Performance: Rails is known for its scalability, as many large-scale applications have been built using Rails. It offers tools and techniques like caching, background processing, and load balancing, which help optimize performance and handle high traffic. Flask, being a micro framework, is better suited for small to medium-sized projects and may require additional configuration and fine-tuning for handling significant traffic.

In Summary, Flask and Rails differ in terms of simplicity, development speed, flexibility, community size, database support, and scalability/performance.