Never hire externally at seed, you're not a mature enough organisation (unless there is a specific individual you have in mind who would be a natural fit for some other reason).
If you're leading the product development you're going to find it difficult to do a CEO-type role. Usually you need someone in the business building solutions to customer problems - that's you - and someone else going out into the world finding customers and selling the product. You don't mention who your cofounder is, but if they're the going-out-selling person then it's more natural for them to be CEO.
If they're not, and you don't have a going-out-selling person, when you really need to figure out how you're going to connect with customers and everything else should come second.
