Which ISO to use?

I wanted to figure out what the best ISO for my Sony NEX-7 is when doing long exposures.

Also I wanted to understand what the best use of my time would be - if shooting at higher ISO would result in better IQ in less time before degrading again due to excessive noise.

I did some test shots with several ISO settings.

I kind of conclude the following, which is perhaps logic if you think about it.

• 8 pictures @ISO3200 / 15sec stacked and averaged
• 4 pictures @ISO1600 / 30 sec stacked and averaged
• 2 pictures @ISO800 / 60 sec stacked and averaged
• 1 picture @ISO400 / 120 sec stacked and averaged

They results all look similar in terms of noise with the 1 exposure looking sharper.

So my question now is: Why should one stack if you could also shoot longer at a lower ISO?

I could understand the advantage of stacking 8 pictures @ISO800 / 1/4 seconds instead of shooting 1 shot @ISO100 / 2 seconds - due to atmospheric movement for example.

But why would somebody prefer stacking 8 pictures @ISO800 / 30 seconds instead of shooting 1 shot  @ISO100 / 4 minutes, assuming tracking is not an issue?

Or am I making a thought error here?

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