I've used both and performance-wise my team didn't see any drastic differences. We switched to EC2 mostly because of RightScale (which now also runs over top o

I've used both and performance-wise my team didn't see any drastic differences. We switched to EC2 mostly because of RightScale (which now also runs over top of RackSpace). It's not cheap, but running on RightScale has made server maintenance and scaling a breeze for us. If this is a pain on your team, I recommend them.