Does Cassandra run better on Amazon EC2 or Rackspace cloud servers?

[Cassy Andra]

My company is looking to develop a software prototype based off Cassandra in
the cloud. We except to run 5 - 10 NoSQL servers for the prototype. I've
read online (Jonathan Ellis was pretty vocal about this) that EC2 has some
I/O issues. Is the general consensus to run Cassandra on EC2 or Rackspace?
What are the pros + cons?