Whirr just uses the AWS APIs to provision a cluster for you, but then you're getting a cluster built on EC2 instances rather than EMR, so you don't pay that EMR overhead. You can choose spot or ondemand instances. If your spots get reaped, I think it would fail pretty similarly to an EMR cluster built on spots. I have no idea how quickly it could provision a 500 node cluster, however.
Ahh, I see - I thought it was about being able to move my EMR jobs to other providers. Running on EC2 without the EMR charge would be nice, and assuming that there's no special casing for spot requests for EMR (I don't know of any, the spot instance requests show up like normal ones) then it may be well able to get me that many instances.
