I agree with Tom. It is a difficult question to answer for all geomaterials (clays, silts, sands, gravels, rocks tailings, etc.). The piezocone has advantage of 3 continuous measurements with depth to aid your evaluation of the ground in question. In comparison, your inconclusive results from SPT-N somewhat restricted in that only 1 number obtained. Plate bearing tests (surface) and screw-plate tests (downhole, also called "compressiometer") can give load-displacement for bearing.