I personally prefer Dotcloud. I don't know if their new pricing model is rolled out all over, but it basically allows for unlimited 'development' sites so getting everything up and running should cost nothing until launch (though it will be memory constrained).

Post-launch, you basically pay for memory usage, so early traction costs very little, and you don't really have to worry TOO much about pricing until you've gotten enough traffic to be considered 'social proof'.

Ignoring cost, I can't necessarily compare it too strictly to Heroku as I've only used Heroku for rails. Anecdotally, Django was one of Dotcloud's reference applications, and Python support has been there for a lot longer.