I heard led was comparable to hps in terms of ... Well everything yield, denseness etc..
The old blurple LED was garbage; inefficient, lacking spectrum and unreliable.
The new COB LED chips are a different breed of cat entirely, and these can be made to be more efficent than any other artificial light source. While it's costly to get those kind of numbers, 56% efficiency is relatively affordable, while being at least thirty percent more efficient than anything with a light bulb. Spectrum is fantastic, heat is minimal, and with minimal care they last forever.
I've seen quite a few grows with them now, and I'm convinced enough to plunk down a very substantial amount of cash to have a large array built for testing.