a) are you good at selling?
b) can you afford to hire someone?
c) do you have time to sell?
Until you can afford to and literally don't have the time too, my gut says do it solo.
I agree with Kelly, if you can sell, and are comfortable selling, then see if you can do it alone if not, hiring a great sales/ marketing person to handle that role while you manage the agency, could be the best move. I have benefited before from working with a great sales team that are much better at their craft than i, while as a designer i can concentrate on delivering quality product.