I've tested both Rackspace Cloud Servers and Amazon EC2, and am currently using Rackspace Cloud Servers in production for my site. There are a number of factors that influenced this decision:
- Support is definitely a factor. I've found the support reps at Rackspace to be really helpful, and in cases, depending who you speak to there, they sometimes go above the call of duty. I had a rep help me setup configure haproxy for load balancing while on their standard support (not managed).
- I've found setup to be simpler. I feel this is an entirely subjective area, but in setting up a new server, I typically just want a clean Ubuntu box. Having community made instances are nice, but not something I need or use, so selecting an AMI just seems to make it more confusing.<p>Overall, I'd say it's very much based on what you're doing. My site is quite simple, so the additional services Amazon offers just complicate the setup, and Rackspace wins in ease of use. If you're startup requires high amounts of memory, cpu or disk space, then Amazon's the winner. I've found scaling Rackspace also to be a lot simpler. When my site's been featured on Mashable, Fast Company, GigaOm, etc., in a couple clicks (even from my iPhone), I can scale to a bigger box.