I work primarily in Django, so I luckily don't have to care very much about which one I'm using. That said, I usually prototype / develop against MySQL, just because it is what I'm most familiar with, and deploy against PostgreSQL, because I believe it to be more stable (when unattended).

Actually, if I'm being completely honest, MOST of what I deploy to is a mix of Postgres and Oracle, but that wasn't asked, and I don't recommend Oracle to anybody that isn't specifically selling on-prem solutions for Enterprise or government customers.