The key to keeping a low profile has nothing to due with LED nor HPS, imho. They make low wattage HPS and very high wattage LED lights with a lot of heat and loud fans. The key to keeping a low profile is very good security regardless of what kind of light people choose.
Mother Nature is often a limiting factor. You *can* beat the Sun, except when it comes to price. Plasma lights have potential but I'm not switching to plasma yet.
There is a theory, that we've talked about on the forums, that suggests that the IR spectrum may be why LED lights lack the yield that HID lights can produce. Some have suggested that raising the ambient temps may help LED lights yield better since most of the LED lights lack the IR spectrum.
Noise is certainly one of the security factors that I was talking about. HID cooling doesn't have to be excessively loud and not all noise draws suspicion.
LED lights are better for some crops than others, obviously. I doubt that they're growing mj in space...yet. LED lights may very well take the crown, in the future, but not today.
We were using it as medicine long before it was legalized for medical reasons in California, patients shouldn't let a law keep them from relief. It's still illegal on the federal level here.
The point still stands; you can't expect top-shelf results by skimping on light and not enough w/sqft is skimping. Suggesting that small light (300w) will properly cover a large space (4x4) is skimping (18.75w/sqft) and not based on real world grow results. CLW suggests 40w/sqft for flowering with their 5w diode LED lights and I'd suggest that even that is exaggerating a bit if you want top-shelf results. You'll certainly do better with 40 than 20 or 30 but even with higher numbers HPS is still king...for now. I've yet to see any LED grow that beats top HID grows.
I'd love to be proven wrong though...