There are 3 reasons I would choose to use AppEngine
1. Software is super simple and working with servers in the future is a no go.
2. Software is going to have 100+ millions users overnight. Will still need to be tweaked and optimized to handle the traffic on AppEngine. I have never seen anyone with this crystal ball but who knows you may have one.
3. Google integration is at the core of the software. AppEngine is really easy to do things like authentication with a Google account. Using Google storage or say the Google SQL service would greatly benefit from the software running inside the Google network.