In the case of a 103 engine in good condition, the standard Bosch copper plug will run better and give a better idle. The reason is that the center electrode of a copper plug is much larger than same on a platinum plug. This is because the platinum is such an expensive material. The wider electrode makes a fatter spark, thus the improvement.

In the case of an oil fouling engine, the platinum will be more resistant to the oil fouling. After you replace the valve seals, go back to the copper.

Good luck,