Don't use AppEngine. I migrated Homejoy off of AppEngine in 2014 and I can't recommend the platform for any reason whatsoever. It's slow, expensive, not based on OSS, and not particularly stable. Bits of it (urlfetch, search, etc) would go down for hours at a time and you're basically hosed until they fix it.
I think Heroku is usable with a similar level of effort -- and you have a much easier migration path to something else if you're not using Google's proprietary datastore IMO.
I think Heroku is usable with a similar level of effort -- and you have a much easier migration path to something else if you're not using Google's proprietary datastore IMO.