If you're going to go the Amazon route then you absolutely need to keep an eye on billing, and set up alerts so that any DDoS which caused a spike in your costs would be caught as soon as possible.
I was burnt by this in the first 48 hours of using Amazon DNS. Very unlucky I guess... I'm amazed they still bill for DDOS traffic, or even traffic from black-listed IPs. It seems many of their competitors don't.