If you’re happy paying a few dollars per hour for a managed solution, don’t need more than 2000-3000 read/write operations per second on average, stick with DynamoDB. If you need “extreme performance” and/or more advanced setups like cross-region replication it’s likely more beneficial to manage your own Cassandra cluster.
The AWS c3.xlarge beats out the group, showing that for this workload using the smaller instance type costs less per operation.