As I was saying in another thread you can only throw down so much light on a plant before it has negative results. This can be accomplished with HPS or LED. LED does cost more up front, but at current efficiency it basically breaks even over 5 years. In the next couple years as new emitters are introduced the overall cost -vs- time will swing towards LED.
So light intensity, and penetration isn't going to be an issue in the near future. The only thing that will be left is quality of light. 1500 PPFD of HPS or HPS/CMH -vs- 1500 PPFD of LED is what it will come down to. It's a good bet that LED will come out on top, with energy savings to boot. This kind of testing hasn't been done as far as I know, but would represent the fairest comparison between the two.