Getting started with Heroku is very easy. It's a very straightforward procedure and a beginner can set up their first app in two minutes. Often it's just a matter of a couple of git commands and it's all set up and running. The official Heroku docmentation also helps a lot.
Heroku is one of the oldest PaaS providers. The fact that it's been around for such a long time means that it had a lot of time to mature over the years. There's also a massive number of articles, guides and tutorials on Heroku out there for beginners and advanced users.
If you have not enabled Preboot it takes some time for the dynos to restart and while they are restarting the application is completely offline. Which means that you can lose precious seconds of application time.
The deployment process with AppFog is generally nice and easy because of its CLI. Unofrtunately the only downside of that process is the fact that it takes too long. A deployment to AppFog genereally takes up to 40 seconds, which should be fine if you are deploying once a week, but when you deploy...
Since Convox itself is a free and open source software, you do not have to pay for using it. Developers only have to pay for the cloud hosting itself (in this case AWS instance) where Convox will be deployed.