I mean, they are completely different. MongoDB is more-or-less a traditional RDBMS with automated failover and trying to staple on more advanced features. Cassandra is a masterless DynamoDB-ish database with features like hinted handoffs. You really need to know how consistency and distributed systems work if you're looking to pick Cassandra. It's a great implementation, you just can't compare it to MySQL/Postgres/etc like you can with Mongo