The result? The High Pressure Sodium produced slightly more weight, with a denser bud structure. Otherwise they were comparable, and the LED did quite well.
The lights have improved to something close to equity. However, HPS prices have fallen to a stable price point which is considerably lower per watt than the high end LEDs. On the basis of cost-to-begin and the initial cost of an indoor system, HPS wins easily.
But, the heat produced by the HPS lighting must be removed from the plants, and the air around the plants conditioned in various ways, and these costs are significant. The cost of overheated plants is also significant. Delayed growth, collapse of potency, hermaphrodism and male flowers in the room, vulnerability to pests, greater pest vigor and breeding rates (especially spider mites), and other results of excess heat are among the biggest threats to profit and quality the indoor grower faces. And the cost of the equipment needed to offset heat, from ducting and fans and filters, to air conditioners, and humidifiers and dehumidifiers has to be added to the initial setup costs.
Of course, one of the primary reasons for the drive for better LEDs these past years has been the cost of electricity. Generally speaking, LEDs cost half or more less than HPS to power. As a necessary monthly expense, saving that much on your electric bill is significant. Combine with that the cost saved on air-conditioning and cooling power.
Combining the two types of lighting is a solution to consider.
Of course, High Intensity Ceramic Metal Halide outperforms them both.