I don't really see a fundamental difference except that Google's offering is more opaque and less flexible. Instead of "your instance got killed, but you can restart it if you're willing to pay $x/hour", you get "your instance got killed, tough luck."
EC2's spot market fluctuates based on supply and demand, and there's no reason to think the same forces won't apply to GCE.
It's not really that different: If your preemptible GCE instance gets killed, you can try restarting it as a 'normal' (non-preemptible) instance. But in Google's case, both prices will be predictable.
EC2's spot market fluctuates based on supply and demand, and there's no reason to think the same forces won't apply to GCE.