I may actually just start with that. From what I can tell, you get a much much larger database than Heroku, but with a lower bandwidth cap (although 50GB vs 2TB for my purposes is indistinguishable). AppFog also claims it has the "Fastest servers available on any infrastructure" (for what it's worth).

Unless I'm missing something, it sounds like a much better deal. Then again, I'm talking about two free services, but it would be nice to be able to avoid upgrading to paid for as long as possible.