Q. Which is more important for a concrete countertop: reinforcement or a strong concrete mix design?
A. If you mean reinforcement to provide tensile strength that allows a countertop to carry loads, the reinforcement would be less important. It's rare that a countertop bridges any significant distance. Cast-in-place countertops rest on plywood formwork that is typically supported every couple of feet by the cabinets below. Precast countertops are supported by the vertical walls of cabinet units and by wood runners along the back and front face of cabinets.
The most significant force on a countertop is shrinkage. Adding structural-rated fibers to a mix is probably the best insurance for preventing cracks. But contractors use a wide variety of steel mesh products and small diameter rebar to counteract shrinkage forces—particularly at the corners around a sink.
The best concrete mix for countertops is one with a low water-cement ratio to reduce the amount of shrinkage. Finding ways to reduce the total amount of cementitious content will help reduce shrinkage, too. You should also produce strong concrete that isn't very porous.