Since AppEngine runs Java, that means it can run JRuby, which also means it can run Rails. Use it if you like. There are a couple of special considerations when using Rails on App Engine, although I am not familiar with them. Finally, you should remember that you're stuck with their datastore, so you can't use ActiveRecord (at least as far as I know, there may be a gem that lets you)

Python should be easier to pick up. I knew very little Python before using GAE, looking back there was hardly a learning curve. It's almost like writing pseudo-code. (Though somehow not as fun to code as Ruby, for me..)

I don't know RoR but Ruby is similar to Python from what I've seen.