I've used both Engine Yard Cloud and Heroku. They're both great and have their pros and cons. Both are also powered by Amazon EC2.
The only major downside with Heroku is if there is any downtime within their infrastructure, your app will also go down. This has happened a couple of times in the last 3-4 months, and it took a lot of big sites down.
Engine Yard Cloud setups are self-contained and can be locked down to whatever setup, cloud version you're happy with. If they update their cloud setup, you don't have to update your instance anymore.
It also seems like you can get more "bang for your buck" with EY once you start needing more computing/db power
The only major downside with Heroku is if there is any downtime within their infrastructure, your app will also go down. This has happened a couple of times in the last 3-4 months, and it took a lot of big sites down.
Engine Yard Cloud setups are self-contained and can be locked down to whatever setup, cloud version you're happy with. If they update their cloud setup, you don't have to update your instance anymore.
It also seems like you can get more "bang for your buck" with EY once you start needing more computing/db power