Cloud and clear and orange top sphere:
- Cloud is more into big room with less bounce wall or ceiling, so the cloud would diffuse the light for you.
- Clear is more into small room with bouncing wall or ceiling plus alittle of diffusion/reflection from the clear doom.
- Orange top I guess it is similar to the color gel, the cloud doom do the diffusion and the orange top do the WB correction from the ceiling bounce. Which without a ceiling, the orange top would do less of a job.
I do not own one, but I did look into it and trying to get one. Afterward, I know I am going to do off camera with boxes or umbrella, so I just forget about the doom. But I do have the on camera Gary Fong diffuser, it does work great before I got my SB-900.
Edit:
I think they have 2 more opional gears that's (#1) shiny silver and it covers half of the doom (where the LCD position from the camera), it works like the bounce card but with diffusion from the doom. (#2) Shiny Silver "top" with a egg sized hole in the middle of the top, it do abit of ceiling bounce and the silver top stop some of the light going up and bounce back to the front. In the other word, less ceiling bounce and more light in the front of the camera.
2nd Edit:
As you said you will do club photog with the doom, without knowing how big the club will be I suggest you use the cloud sphere. You don't have to take care of too much bounces stuff, plus, if light isn't enough from your flash using the doom with the flash head facing ceiling, you can turn the flash head to the people with the doom on. That way you get more light, but alittle less diffusion, at least you will have enough juice if you need them.
Hope it helps.
Kcc