If I remember correctly the light cannon is a HID where the Shockwave is just LED. On my trip to Bonaire, I used a UK C8 eLED which I think is pretty equal to the Shockwave LED. On a night dive my dive buddy was using some kind of HID that was about the same cost as the UK you mentioned. His light beam was much more focused so it cut through the mess. We were fairly shallow and the water was a little rough kicking up sand. His cut through better, but mine was not terribly worse. Mine did a much better job of overall illumination where his did better at penetrating through the sand. Overall I was very happy with the performance of my new light as I had no idea what to expect. For a less than half the price of his, I had a great light that worked well. Although it didn't light up as much, the lower output setting worked better for cutting through on mine. Headlights are the same in fog.
As for cold water you probably won't notice a difference. The more kicked up silt, the better off you will be with a HID. If it's just night dives, get the LED lamp and save yourself a couple hundred.