Heroku or EngineYard? Which RoR hosting provider is better for a B2B app?

We are working on a B2B application and need email processing in/out, sub-domain mapping for each customer account etc. etc. (basecamp type of app architecturally but for a completely different domain) We like the simplicity of Heroku but when we looked at their customer list - could not find any serious B2B apps listed. I've read nice things about Engine Yard and have setup a test account running but their documentation is not very clear. I have read almost every post on HN comparing these two but still unable to decide which way to go. Any suggestions ? Thanks.