Up then down (if you are connecting A with B, start to finish). The curvature of the earth drops 8" in a mile; but would you then be going up 8" if you went in reverse? No. If gravity is still in play, it would have no accelerating effect credited to that 8" drop. It would be constantly "felt" along the arc of the frictionless surface (also assuming the earth's mass is perfectly apportioned and the center of mass is the true center of the earth and you were consistently the same radius from that center of mass). So it wouldn't have the effect of accelerate you because it would be applied perpendicular to the surface, not along the surface.