You get more than just the instance. We take care of keeping the servers alive, patching them and such. We also load-balance your traffic, which is usually something you have to add if you're picking another provider. Finally, there's what people like most about AppHarbor, the ability to push code to Bitbucket or GitHub and have it build and deploy, but only if everything checks out. Rackspace is closer to the infrastructure layer. Some prefer that, but I bet most will prefer the convenience of a platform.

Great, thanks for the reply. I will certainly give it a go.