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.
Heroku has a vast list of plugins and services that can be added to an instance. These plugins cover things from databases to email systems. This remove the task of having to install services and setting them up manually. Heroku does it all for you.
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.
OpenShift seems to rely more on written documentation and on the community to solve any problem users may have. The forums and IRC channel are active and very helpful, but the official customer support could be better.
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.