Drizzle is not a drop-in replacement for MySQL. The only real option if you want to replace MySQL without having to change your application code is MariaDB. Drizzle is fine for new projects, but you'd have to massively alter an existing site to get it to work with Drizzle.

I don't think it's fair to say MariaDB is the only real option. The CTO of the company I work at seems to be a pretty firm advocate for Percona.