We've used Rackspace Cloud and Linode for several years and found Rackspace Cloud performance to be significantly worse than Linode for CPU and IO. Like, not even close. Prices are about the same. Support experiences are also similar (quick, competent responses from both sides).
The only reason we keep the Rackspace stuff around is because we want to have a relatively wide separation between our production webservices and our website / monitoring so that if one goes down that we can announce / detect that from a wholly separate network.