There's 2 arguments going on simultaneously, and people constantly mix up each other's arguments. My thoughts on each are that:
1) LED is more efficient than HPS. It's hard to argue this when you look at what people are routinely yielding under the more efficient builds and it puts out more light per watt.
2) HPS makes more financial sense than LED in most circumstances, works just as well as LED, and doesn't sacrifice as much opportunity cost up front. You can setup a lot bigger footprint earlier on, which means more money. HPS are still very efficient.
If someone was asking me what light to get for a 4x4, I'd probably recommend a 600W or a 1000W HPS, but if someone was asking for what to get for a 2x4 or smaller, I think COB starts to make more sense.