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In general, lower ISO results in less noise than higher ISO. Similarly, shorter exposures result in less noise and hot pixels than longer exposures. I'm sure there's an cross-over point somewhere depending on ambient temperature. If you're stacking multiple subs, then noise becomes less of a concern.
I remember reading an article that someone wrote where they systematically tested ISO 100, ISO 200, etc with multiple stacked long exposures on a Canon dSLR. From memory, I think the bottom line was that ISO 800 and ISO 1600 was the best option for them (trading off exposure time versus noise, etc). I can't recall the link, sorry.
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