[carbon8]

Engine Yard is more like opinionated configuration management. It allocates and configures EC2 instances that you can log into like normal. The software stack is HAProxy, nginx, unicorn, etc, and customizable through the web interface and/or chef.

[kawsper]

Is there a reason for using both HAproxy and Nginx?

[photomattmills]

HAproxy is a more efficient load balancer for really high scale apps. That said, only about 5% of people would see a difference in load/memory usage. Source: I work for EY.