Amazon's actual ELB IPs can handle relatively little traffic, "prewarming" is required in order to add more ELB instances--during a DDOS attack, you'll be overwhelmed almost instantly. Route 53 uses DNS round robin, which is trivial to bypass if you're planning on a DDOS attack (by targeting a specific IP). Google actually gives you an anycast IP, so they're a better option.
All that being said: the idea that only ingress traffic matters during a DDOS attack isn't quite right. If the connections are legitimate, you either need to be able to detect the attack attempts (requires expensive coordination and mitigation techniques, especially if the attack is much larger than what a single NIC can handle) or actually serve back the content (which will make your egress skyrocket).
