ASK HN: Whats the best AWS server to host RoR app

Am using AWS E2C to host our RoR app using Opswork. Am currently using a c1.medium instance for our production app. In a few days AWS is cutting down prices and I would want to move to a new instance. My question is What should I look at when choosing a new instance for a RoR app? lots of RAM ? high CPU usage? from what I can tell in my new relic metrics I should go for a large RAM instance. But am not sure.

Our app is a consumer app with medium traffic and I run everything using unicorn and nginx.

Any help, adivice and guidance will be appreciated

thanks