I've conducted extensive benchmarking of rackspace cloud, ec2 and about 38 other iaas providers. Rackspace cloud is definitely not faster than ec2 by a long shot. Rackspace cloud utilizes homogonous infrastructure, AMD 2374 to be exact. All instance sizes are burstable, so you typically get about the same CPU resources on a 1GB instance as you do on a 16GB instance according to our benchmark results. Ec2 on the other hand scales CPU much better all the way to dedicated dual quad X5570s with the cc1.4xlarge. Both this and the rackspace sponsored bitsource study compared an ec2 m1.small. Any study that does this should be immediately discounted as that is about the worst possible performing ec2 instance size. Adrian cockroft from Netflix refers to these types of studies as benchmarketing. They do not accurately depict the performance capabilities of ec2.
http://blog.cloudharmony.com/2010/05/what-is-ecu-cpu-benchma...