Well let me put it this way. As a non-technical founder, your chances of identifying a great agency and that agency being willing to work for you are one or two orders of magnitude greater than your ability to identify a great developer and convincing him to be your co-founder.

Of course if you have an inherently interesting idea, a Jobsian charisma, or an uncanny nose for talent, then that can shift things in your favor. But I can say with some authority that the number of new entrepreneurs these days far outweighs the number of talented developers who are entertaining new startup job offers.