Well I see my fan club got here before me. :D Thanks all !
Just to correct ahar (sorry)
There is a substantial difference in Lumens on the range.
10W = 510 lumens
18W = 1300 lumens
21W = 1500 lumens
24W = 1800 lumens
24W is a video light as has been said, the 18W is the most popular light, having the best colour temp (steel blue white light).
The only 21W light available is the remote ballast unit.
Also a point to bear in mind the H lights have the (some say) fragile WA bulbs in. I know I'm biased
Hope that helps some in your deliberation, if not give me a call and I'll see if I can help you through the maze.
Best