Given this came up reasonably recently, I thought some would find this interesting. I didn't write it, not affiliated with anyone, I don't use either, but thought it was interesting.
http://www.thebitsource.com/2010/01/11/rackspace-cloud-servers-versus-amazon-ec2-performance-analysis/
-sc
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Sean Chittenden
sean(at)chittenden(dot)org