Kind of silly that you wouldn't use MongoDB yet you'd use GAE. GAE is a turd, Google has been struggling to compete in the cloud business and failed to come out with any value. You get locked into a Heroku-like platform with awful performance, awful stability, and arcane quotas.
http://www.carlosble.com/2010/11/goodbye-google-app-engine-g...
http://3.14.by/en/read/why-google-appengine-sucks