This is true, generally all Rackspace Cloud servers perform about the same in terms of CPU and disk IO, so the small Rackspace instances will outperform small EC2 instances. However, they don't scale well and on the high end, they under-perform relative to comparable EC2 instance. Here is some sample data validating this (these web service links will provide XML formatted benchmark results):

1GB Rackspace vs EC2 Micro - CPU Performance: http://cloudharmony.com/ws/getServerBenchmarkResults?serverI...

1GB Rackspace vs EC2 Micro - IO Performance (using EC2 EBS): http://cloudharmony.com/ws/getServerBenchmarkResults?serverI...

16GB Rackspace vs EC2 cc.4xlarge - CPU Performance: http://cloudharmony.com/ws/getServerBenchmarkResults?serverI...

16GB Rackspace vs EC2 cc.4xlarge - IO Performance: http://cloudharmony.com/ws/getServerBenchmarkResults?serverI...