Smoque creates more of a hazy effect, there is less halation concentrated right around the source, it's more spread out, and Smoque doesn't impact sharpness much. But the effect tends to disappear when something temporarily blocks the light source in the frame.
Double Fog is more like a Regular Fog, you get the blue-ish glow right around light sources. It's just a bit less soft than Fogs, but a bit milkier.
I would use the lightest grade, a #1/8 Double Fog, maybe a #1/4 at the heaviest. I just used an old #1/4 Regular Fog for a flashback scene and though that would have been considered very subtle by 1970's standards, it was fairly obvious to me (strange thing is that "Star Trek IV: The Voyage Home" was mostly shot with a #1/2 Fog and it doesn't look that heavy.)
The older design of the Fogs (and maybe the Double Fogs and Low Cons) is that the dispersion of mist particles in the laminate doesn't have enough clear spaces between particles to allow a sharp image to pass through... so depending on the amount of halation in the shot, there can be some loss of sharpness compared to a ProMist for example. But if you use a #1/8 Double Fog, it shouldn't matter because the filter is so light to begin with.