The pros and cons depend entirely on your application. You've probably already played out a plethora of scenarios in your mind to explore the pros and cons of using the cloud, Amazon EC2 to be specific, for your project. Following are a few more genuine concerns that I would suggest you to look into.
Home access to the servers: Administrators with access to the EC2 servers will be able to make changes to your project environment from home. Furthermore, the key pairs for your EC2 servers might remain available to the employees who leave or get excluded from the project making home access to servers seem like a major concern.
Validating security: With the on-premise environment, you can be more than 90% certain that all your servers and resources have been covered behind a firewall but with cloud, it becomes a little difficult to ensure such controls since its, more or less, just relying on the word of your cloud provider.