Well, spot varies. Sometimes it's low, but the price varies based on demand. Using prices as of right now for example, Currently in California, c3.8xl instances are quite popular, running $1.68 an hour on an instance that's normally 1.912 an hour, roughly a 13% discount. 70% strikes me as better than 13%, especially when it's predictable. (and 70% below an already substantially lower hourly price...)
The long-run price for a c3.8xlarge in California is somewhere between $0.40 and $0.50, though you're correct that right now they're $1.68.
California tends to be more expensive than other AWS regions, though (I can't remember the reason - perhaps just availability?). If you're in us-east-1 the price sticks around $0.20 per hour, and eu-west-1 is rock-solid at $0.32 per hour.
If you have work that can be done on spots it can be a good idea to make it region-agnostic so that you can take advantage of better prices for different instances in different regions.
The idea with Google is that you don't need to worry about which instances you run, at which price, and in which region.
For example, with Google you don't need to get a specialized instance type to use it's monster-fast Local SSD - just use the same instance types. This alone should simplify use of Preemptible VMs.
[1] http://storage.googleapis.com/pricingcomparator.appspot.com/...
[2] http://storage.googleapis.com/pricingcomparator.appspot.com/...
disclaimer: I work at Google on Google Cloud Platform