LED seems good until you get it in the water, it seems that it dosn't penetrate dark water very good,HID is the dogs nad's i have a pro 6 witha 18 watt head, one of the best bits of dive kit i have bought ( apart from a VR3)
Power is good, colour temperature is better, this is why a 15W HID (around 62~ 6800 deg K) is brighter than a 50W halogen (@ 4700 deg K) in poor viz. I have a Greenforce TRISTAR Diamond 3 LED torch which has a colour temperature of 58~6200 deg. K (degrees Kelvin for what it is worth). This particular torch has the equivelant light intensity of a 15W HID, but, does not quite have the penetration. I have tried the equivelant HID head and to be honest I want one, it is definatly better. But, the LED torch will do a weeks livaboard without a recharge., solution get both, can't afford that so the HID will wait.................... But not too long.
If it was me, I would still go for the LED, but as a backup to the HID. HID has it's irritations, warm up time, bulb life etc, if it is a one off purchase and money is a major issue then the LED will last for ever, well at least a very long time, the only consumable being a bit of silicon grease every now and again.
It is comparitivly (aginst my previous Sunlight C4), bloody fantastic, friends refer to it as the thermic lance, so it is fairly bright.