AWS vs Heroku vs something else for scalable platform?
Recently we have done a very good comparison between AWS and Heroku and we decided to move to Heroku, here is the detail of this http://www.confiz.com/blog/tech-session/selecting-the-right-cloud-platform/
If you're on Python, you can try Google App Engine.
Migrating the Python app from one platform to another isn't too difficult once you get the past the learning curve as to what features is (not) available. GAE offers datastore, memcache, blobstore plus a few other goodies like dJango and Jinja (templating). Worth checking the Python start page and it didn't take me long to integrate it into Facebook and Dropbox too.
Heroku. Because you're a start-up, keep things lean and it doesn't get leaner than almost free (with 1 dyno + small shared DB). Spend time building your product, not on the infrastructure. You don't want to be installing patches when you should be talking to customers. Heroku is also flexible and allows you to scale up 'dynos' as your traffic increases so no worries about growing there. Heroku won't scale automatically for you, though, so do your own server monitoring. Heroku add-ons are also nice.