Cake flour or all-purpose for shortbread?
What you really need is "in the middle". Shortbread cookies are not very different from shortbread pie crust, they use the same principle, but a different shape. So the optimal flour would be pastry flour, at 6-7% gluten, which is between cake flour (4-5%) and AP (7-8%). 

If you are not aiming for five-star-perfection according to century-old traditional tastes, either of your flours will do well. AP is probably a bit safer, in the sense that the cookies won't fall apart too easily.