Firebird can do both, though.

I often see this quoted but I don't see how they compete at all? Or am I just not getting a joke?

I have seen a lot of enthusiasm for using sqlite instead of postgres for django/laravel/rails type projects, or just using it for your tests. I think both of these opinions are over valued since it's easier to learn postgres once then to learn sqlite then migrate to postgres.

The real developer cost of "install postgres, createuser, createdb" is basically nothing.

I see the cost of ever using sqlite (coupling my tests/and/or app, migrating as soon as you have 2 users) as much higher.

Sqlite does seem like a safe, solid, and sane default for the embedded/fopen people though.