In my own informal experience, running identical CPU-bound tasks on Rackspace and EC2, it's just a lot easier to get good performance on Rackspace. Instance si

In my own informal experience, running identical CPU-bound tasks on Rackspace and EC2, it's just a lot easier to get good performance on Rackspace. Instance size on Rackspace does affect performance (as their FAQ explains), but it affects minimum guaranteed performance; there's bursting up to the capacity of the entire machine you have a slice of. I know there's something unsettling about relying on excess capacity, but in practice any Rackspace instance will tend to beat any but the most expensive EC2 instance in terms of raw CPU. The sweet spot of EC2 really is offline processing where hundreds or thousands of machines are spun up by software. If you want to manually fire up a small but flexible number of fast Linux machines, I would recommend Rackspace.