Putting the gauge on the valve allows air from the tire to enter the gauge. On high pressure/low volumne tires this can effect the pressure slightly so if it was 100 psi before putting the gauge on it will be slightly less afterwards.
My guess is that the Smart Gauge is more accurate but the only way to tell would be to test it against a gauge that is properly calibrated. As to the accuracy of the Road Morph gauge, pump the tires to a certain pressure on it and check it with the gauge.