The spectrum of a high pressure sodium lamp:
Unlike low pressure sodium bulbs, the blue spectrum is intensified in a high pressure sodium lamp, making it not just exceptionally effective for flowering as the left of the spectrum indicates, but also highly effective for vegetative growth. Metal halide lamps, while offering a more blue oriented spectrum, are almost half century old technology, consuming more electricity and having a generally shorter life span.
This is not only due to their lower wattage efficiency but their tendency towards something called "cycling", wherein the initial voltage required to start the lamp actually rises over time to the point of exceeding the ballast's abilities, rendering the bulb effectively useless.