Assuming you're both talking about Google Cloud SQL and AWS RDS, then with RDS you can actually choose between MySQL, PostgreSQL, SQL Server, Oracle, or Amazon Aurora which is supposed to be compatible with MySQL 5.6. Google Cloud SQL only seems to mention MySQL.
For scalability, RDS gives you up to 3TB storage (1TB for SQL Server, 64TB for Aurora) and DB instances with up to 244GB RAM, 32 vCPUs and 10Gigabit network, whereas it looks like Google Cloud SQL is 100GB and 16GB of RAM. It looks like Cloud SQL provides managed replication/failover as standard; with AWS this is optional, but simple to set up. (I use RDS; never tried Google Cloud SQL.)
(I have no connection with either company, beyond being a customer.)
Amazon RDS for Aurora is what I was thinking of as being similar to Google's Cloud SQL... they're both mySQL API layers over the top of their BigTable/distributed backends.
I was actually thinking of Amazon RDS for Aurora[1], which seems to be a mySQL api layer over Amazon's BigTable backend. and similar to Google Cloud SQL.
