No room to place the tubes horizontally?

Personally, I'd rather go with the 400w HPS. I had one in my 2 x 4 x 6" tall closet but you do need to vent the heat, which makes stealth more difficult. But regardless of set-up, constant fresh air is as important as venting heat out.

I have no idea how close the LED's need to be, but the effective HPS light would extend up to 3 feet or so, which comes in handy if the canopy is uneven. Versitility and proven performance outweigh my curiosity about LED's.