1. Scalability: Amazon S3 is a highly scalable storage service that allows for infinite storage capacity. It can handle large amounts of data and is designed to handle high traffic loads. On the other hand, Memcached is an in-memory caching system that is primarily used for speeding up database-driven websites. While it can also handle a significant amount of data, its scalability is limited by the available memory on the server it is running on.

2. Data Persistence: Amazon S3 is a fully durable storage service, meaning that once data is written to S3, it is highly resilient and durable even in the event of hardware failures. It provides automatic backup and replication mechanisms to ensure data durability. In contrast, Memcached is an in-memory caching system and does not provide built-in mechanisms for data persistence. If the server running Memcached were to fail, the cached data would be lost.

3. Data Access: Amazon S3 provides RESTful APIs to access and manipulate data stored in it. This makes it easy to integrate with various applications and services. Also, S3 can be accessed from anywhere with an internet connection. On the other hand, Memcached does not provide a RESTful interface. Instead, it provides a simple key-value interface based on the Memcached protocol. It is typically accessed locally or within a network, making it more suited for caching data within a single application or server.

4. Data Retrieval Speed: Amazon S3 is optimized for storing and retrieving large files and can handle a high throughput of data requests. However, it may have higher latency compared to Memcached when it comes to retrieving small, frequently accessed data, as S3 has to perform disk operations to retrieve the data. In contrast, Memcached is an in-memory cache and can provide very low latency for retrieving frequently accessed data, making it ideal for improving website performance.

5. Cost Model: Amazon S3 follows a pay-as-you-go pricing model, where users are charged for the amount of data stored and the bandwidth used for data transfers. This allows for flexibility and cost control based on actual usage. On the other hand, Memcached is an open-source software and does not have any direct costs associated with it. However, the cost of deploying and maintaining the servers running Memcached needs to be considered.

6. Use Cases: Amazon S3 is commonly used for storing and serving static files, hosting websites, backup and restore, and large-scale data analytics. It is preferred for scenarios that require durability, availability, scalability, and high throughput. Memcached, on the other hand, is primarily used for caching frequently accessed data to improve website performance. It is suited for scenarios where low-latency data access is critical, such as session management, database query caching, and dynamic content generation.

In summary, Amazon S3 is a highly scalable and durable storage service optimized for large-scale data storage and retrieval, while Memcached is an in-memory caching system focused on improving website performance by caching frequently accessed data. The choice between the two depends on the specific use case and requirements of the application or website.