All the innovation at AWS is amazing. If they every stop charging by the Gigabyte for bandwidth and move to a flat model then I would be tempted to switch to them for all my sites.
Why would they do that? It makes sense to charge per gb. It gets people using them when they are small and its cheap to use S3/EC2 and then as they scale up its easier to just stick with AWS.
Peering links are charge on a consumed bandwidth basis (in a weird sliding scale fashion depending on your balance of egress vs ingress. The closer to 1:1 the better) rather than the fixed fee you pay your ISP.
I used to work for an ISP that would juggle which peering links the usenet servers were favouring to make things more favourable :D