Founder or CEO title for startup

Depending on the context I'd say. Formally, Founder & CEO ;)

At some point, Founder is often associated with the most early-stage startups. One rarely refers to Jeff Bezos or Mark Zukerberg as Founder; they are mostly referred to as CEO. So, if you want to be taken seriosly, it's often better to introduce yourself as a CEO. (Of course, this depends on the current stage of your project and your objectives)