Those aren't exactly comparable products - also, is it obvious which one sells better? Is it well known that amazon postgres sells like hotcakes?

Odd comparison, Google has supported MySQL for a long time, as has AWS.

Dynamo might be a slightly closer comparison

Google Cloud SQL supports postgres as well.

that's kind of my point

databases people know and use already (eg in fun projects) are easier to sell

Google Cloud SQL and AWS RDS now both support PostgreSQL.