Minor quote on basic comparison of MongoDB vs DynamoDB from Amazon:

EnginesDynamoDBMongoDB
Data Model Key-value with JSON support. Up to 400 kb record size. Limited data type support
  • JSON-like document
  • Up to 16 mb record size
QueryingKey-value queriesQuery & analyze data in multiple ways — by single keys, ranges, faceted search, graph traversals, and geospatial queries through to complex aggregations

In both cases they perfectly suite for big data storage. Both offer cloud-based (AWS, and Mongo Atlas) and local storage solutions.

Positive feedback on DynamoDB and its pricing policy  (pricing calculator):

Amazon lets you buy operations per second capability rather than CPU hours or storage space.

This removes a whole lot of complexity for developers who would otherwise need to tune the database configuration, monitor performance levels, ramp up hardware resources when needed.

This provides users a fast and reliable storage space for their needs with costs that scale in direct proportion to the demand.

For now, DynamoDB looks more favourable. The fact that it’s an AWS-only engine doesn’t set any limits. Its overall stack of AWS tools that exists in their infrastructure looks promising.