Selecting Amazon RDS with Reserved Instances provides cost savings over on-demand instance pricing due to the commitment to a specific usage term. This approach is beneficial for databases with predictable performance needs, such as the one described, as it allows the company to plan its capacity and manage costs effectively. Using the on-demand pricing model would be less cost-efficient given the predictable, consistent demand. Although Aurora and DynamoDB provide performance benefits, they might not always translate to cost savings in cases where the workload is stable and the capacity is predictable, making them less optimal for this scenario. Spot instances are not suitable for mission-critical databases due to the possibility of interruption.