Demand is elastic of course, and if you really want to scale in a cost-effective manner you also need to do auto-scaling. As far as I can tell (I have no direct experience with GAE), it's much easier on AWS. It would also be interesting to see if you can scale your pool of webservers from 1 -> 200 faster on AWS or GCE.
The article does quote @cloudpundit, who hits on the true point of the exercise: Relevance of GCE LB load test: With AWS ELB, if you expect a big spike load, must contact AWS support to have ELB pre-warmed to handle load. I would also guess that Amazon is working to improve ELB to behave similarly, especially now that Google's product has less restrictions than theirs.