Both are great, but different. EngineYard provides much more.
For example, I'm an EngineYard customer. EngineYard developers and support people have spent upwards of 100 hours with me discussing more advanced needs for security, scaling, integration, and professional services.
EngineYard's leadership has especially impressed me with Rails 3, JRuby, and Rubinius.
http://www.engineyard.com/company/press/10-06-09-engine-yard...