I would like to add my 2 cents here. According to me Cassandra is different in the following ways :
Cassandra is a distributed key/value store where keys and values can be anything. The focus is more on making writes faster, so if you are storing data with key/value structure, it can work really well.
If you don't want massive scalability, then MongoDB may probably be a better choice. But if you do want great scalability, Cassandra's no-single-point-of-failure architecture will be really helpful. MongoDB's global write lock can be painful at times. In Cassandra you get more fine grained control over how replication works in your app.
If you are storing JSON data, MongoDB would be a suitable solution because it stores data in bson format. This way querying the data becomes easier for you.
Cassandra is written in Java and can store huge datasets in almost SQL.
I am not an expert, but it might help you.
Thanks.