At my last job, I did some evaluation between EC2 and Joyent with a node.js based application. Hands down, Joyent beat EC2 up -- there wasn't an explicit 1:1 correlation anywhere close, as I was seeing the performance with a m1.large go in line with what I was seeing for a small instance at joyent.

When you got right down to it, what killed that plan was that you were still using Amazon S3 for storage; the money that we would have saved in CPU/processor time we would be eaten alive with transferring that data back to S3.