It depends on your application. Every provider has their own stack running in the back, their own policies, and a unique set of existing users. This translates to unique environments that are more or less suitable for certain needs.

My experience with EC2 is that you get more RAM for the buck when compared to Rackspace and others, but IO to disk and CPU is sub-par. As a result, I tend to prefer Rackspace who are also big players in the open source space.

Keep in mind, regardless of any advertising, there is no such thing as guaranteed performance in the cloud. Ever. Every provider oversubscribes ... they do not expect and cannot handle 100% utilization by their subscriber base. Behind the scene, a VPS is sharing all of its resources, always. If your provider does its job right, you'll never experience a drop in performance. Everything will be consistent. But there is no way for that provider to guarantee that. They can experience an unexpected jump in resource usage at any time.

With dedicated hosting, that's not a possibility. It's your hardware. You're the only one using it.