MongoDB and Riak are both popular NoSQL databases, but they have key differences that make them suitable for different use cases.

1. Data Model: MongoDB is a document-oriented database, where data is stored in flexible, JSON-like documents. It supports complex hierarchical structures and nested arrays. On the other hand, Riak is a key-value store, where data is stored as a simple key-value pair. It does not have built-in support for complex data structures like arrays and nested documents.

2. Scalability: MongoDB uses a horizontal scaling approach by allowing data to be distributed across multiple servers in a cluster. It automatically shards data across the cluster, enabling high scalability. Riak, on the other hand, also supports horizontal scaling but uses a distributed approach known as "eventual consistency," where data changes may take some time to propagate across all nodes. This approach ensures high availability and fault tolerance but sacrifices some degree of consistency.

3. Data Durability: MongoDB provides data durability through replica sets, where each data replica is stored on a different server. This ensures that even if one server fails, the data is still available. Riak, on the other hand, offers data durability through a configurable replication factor. It allows users to specify the number of replicas for each object, providing a level of fault tolerance and data availability.

4. Query Language: MongoDB uses a flexible and powerful query language called MongoDB Query Language (MQL). It supports a wide range of queries, including complex aggregations and joins. Riak, on the other hand, does not have a dedicated query language and mainly relies on simple key-based operations. It does not support advanced query functionalities like joins and aggregations out of the box.

5. Consistency Model: MongoDB offers strong consistency guarantees, where data written to the primary replica is immediately visible to all subsequent reads. Riak, on the other hand, uses eventual consistency as its default consistency model, where updates to replicas may take some time to propagate. This approach allows for high availability and fault tolerance but may lead to temporary inconsistencies in data read from different replicas.

6. Community and Ecosystem: MongoDB has a large and thriving community with extensive documentation, tutorials, and a rich ecosystem of third-party tools and libraries. Riak, on the other hand, has a smaller community but still offers good documentation and support. Its ecosystem is not as extensive as MongoDB's, but it provides solid support for distributed systems and scalability.

In Summary, MongoDB and Riak differ in their data model, scalability approaches, data durability mechanisms, query language capabilities, consistency models, and community ecosystems. These differences make each database better suited for specific use cases and requirements.