Did you ever play a game of Civilization and attack an entry-level Warrior with a stack of machine gun infantry?

Well, the warrior has lower upkeep costs. Keep that in mind.

It's not.

The only thing similar about the two is that they both store data and have the letter D in their name. Otherwise they are completely different, Cassandra being a BigTable style database and MongoDB being a document one.

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