One thing I enjoy about Heroku is the option to export code and run a local Rails + Postgres instance if the need comes to fruition. The "escape hatch" is good

One thing I enjoy about Heroku is the option to export code and run a local Rails + Postgres instance if the need comes to fruition. The "escape hatch" is good for any system in which you intend to invest resources, even if you never need to escape. I think AppEngine is a little bit more proprietary, although with some work you could probably move to Python, Django, CouchDB or something similar in an emergency.