You know, I was considering doing some experiments myself in that regard. Its going to vary quite a bit depending on what lights you're testing, the camera and the ambient conditions.
Personally, I prefer to use the minimum ISO (100 on my camera) with a fully open aperture (lowest "f" setting) when taking any pictures at night with my digital camera - this minimizes grain and the ugly chroma noise (from high ISO). For a starting shutter speed, I used manual mode (with the settings above) and adjusted the shutter for what the camera recommended from its metering (full frame metering) on the brightest light I planned to shoot. I then shot both lights and lengthened the shutter a bit to improve exposure for the dimmer light (this will vary of course depending on the comparison, still was loosing a lot of the subtle spill). If you have an "aperture priority" mode, you can also use that to fix the aperture then note what shutter speed the camera decides to use. So, there is some trial and error to it - just shoot for a close approximation to what your eyes see. Its likely that increasing the ISO would help a bit to capture the subtle spill of some lights (and more closely approximate human vision which has far better dynamic range than your average digital camera), so you may want to try that as well depending on your results. I still feel that a low ISO and open aperture is a good starting point. With very bright lights, and depending on the camera, sometimes the aperture will have to be closed a bit to limit internal reflections from the lens (I've had this problem when shooting the moon with my Sony DSC-V3 with telephoto attachment, had to use my smallest aperture in that case - f8), but in most cases you should be fine with a wide aperture.
Guess its not really the answer you're looking for, but one that in my experience is accurate.
