After working for years with both, I'd say that PostgreSQL is much more friendlier to the developer and more pleasant to work with. In any area: documentation, features, error messages, available SQL features, available extensions, available docs, available books.

One tiny example: I prefer to work with databases using CLI interfaces (mysql and psql).

psql CLI is a tool which is pleasant to use, has no bugs in the interface and it even gets improvements from time to time.

mysql CLI is awful to use (e.g. doesn't display long lines properly, has difficulties with history editing, etc) and looks like there wasn't a single improvement since 1996 (I'm sure there were, I just never felt the effect of such improvements).