AWS EC2 or RackSpace [closed]
Did you read http://www.rackspacecloud.com/cloud_hosting_products/servers/compare?
I'm using Rackspace Cloud Servers and Files for 4 months, and there wasn't any downtime and service outages for this time. Support is great. One thing is valuable: Amazon offers his cloud services in Europe too (Ireland), when Rackspace is based USA only.
UPDATE:
Now Rackspace offers Europe-based cloud hosting (servers located at UK): http://www.rackspacecloud.com/blog/2011/01/19/rackspace-launches-cloud-infrastructure-in-europe/
You may want to see if a cloud provider is really needed, as you often can get a shared VPS for much cheaper than what is offered by Amazon or Rackspace. I wrote a blog post about the differences between VPS/VM, Dedicated, and Cloud Servers and recommend you also look at other mid-range providers like Superb.Net instead of just Amazon or Rackspace.
Im my opinion:
If your thinking that you know the number of users that will access the server and that the number of users is predictable and static, then go with your own Rackspace server.
If you think the number of users accessing your server is unpredictable or could grow in an unknown fashion, unpredictably, then Elastic Beanstalk is the way to go.