AWS is an IaaS and Heroku is a PaaS. To be simple, if you have enough time then choose AWS, launch, install and configure whatever you need and save money. Or if you can spend enough money then choose Heroku. They will take care of all the sysadmin's job, and you will just have to concentrate on your application.
To be more detailed, choosing Herko as you are a startup is a better option. Just work on your application rather worrying about the architecture. Heroku takes care of installing necessary patches to your infrastructure. And also Heroku will not scale automatically for you, do your own monitoring and for a startup, I think this would be a better option considering expenditure.