Brent Sordyl's blog

On average, Rackspace Cloud Servers was more than twice as fast as Amazon EC2 at compiling the Linux kernel across all instance sizes. It should be noted that the EC2 small instance skews the average considerably, weighing in at 24 minutes to compile the Linux kernel. This difference comes heavily into play when paying by the hour for compute resources. Even when pricing is per hour, if completing a task such as compiling the Linux kernel takes longer, the pricing will increase in proportion to the amount of available compute resources.
(Link: Rackspace Cloud Servers versus Amazon EC2: Performance Analysis)