[domnit]

S3 is pay per use, but it's for static data hosting. EC2 is pay-per-use virtual machines, but it's at the granularity of machines. If you need a bunch of real, or real-looking servers, that's good, but it's too expensive if what you need is shared hosting.

S3 is cheaper than NFSN, so if you have lots of big files, you can host www.example.com on NFSN and point media.example.com to S3.

(Grain of salt: I haven't used EC2.)