Michael
Some of the answer depends on your budget and your situation. For instance I travel with photo and video gear and I try to get enough in my carry-on so as to have enough to shoot video or photos even if my other gear does not make it. And weight restrictions between strobes, arms etc. also come into play. Though I would love to carry my FX1 in a Gates Housing with a Night Rider or Green Force light rig, it is not practical for me to carry these things.
As an aside, a good overview of types of lights is here
HID and Halogen lights
FWIW the people at Backscatter are very helpful and I think they are exclusive for the Nano (Nope I do not work for them or anything, just buy things from them
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One last thing of course is what type of diving do you do. I tend to go to places that usually have very good visibility and mostly things that do not need extra lights. I do go into some wrecks and also dive Cenotes, so I like some light for these and also to light things under ledges.
Just to give you an idea, if you go here http://www.digideep.com/english/info/html/sony-hdr-hc3-and-gates-housing-review/
(for some reason the link is not working right now for me) I discuss the Hartenberger light a bit.
It will not light up an entire cavern, but it worked for me. As an FYI you will see a shot in a Cenote in the clip - I had two lights with me, the Nano and another. The other bit the dust early in the dive so I only had the Nano, and to conserve battery life I did not turn it all the way up (I was using my video lights also as dive lights - kept one on low power to have light, then would turn them to full power when I wanted more light - well that was the plan, but when one went down, I had to play it more conservative
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If I recall correctly, the shot in the wreck had both lights on, it was when the bad light was working sort of. It’s output was about the same as the Nano. I did not like the light at all because it was inconsistent/buggy, so most of the time I just carried the Nano, and the only time I hooked the other up after the first day was in the Cenotes.
Though it seems Lux ratings are all over the place sometimes, the HC1 is 7 Lux and the HC3 (used in the clip) is 5 Lux
It is not an exact answer, but HTH