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Sorry if it's adressed in the links - but is seems that a few people don't 'get' the effect of stacking on random noise. Signal to noise ratio (where the noise is random and non-stationary) is improved by the square root of the number of stacks. So stacking 9 subs only gives you a threefold improvement in signal to noise. There is a law of diminishing returns with stacking, eventually you reach the point where it doesn't really improve things much. With astro images it is likely to be at N=100 or greater - I've never taken enough to find out.
Longer integrations will have a similar effect on thermal noise, as it is random and will tend to cancel out over time, but the desired signal strength will be increased linearly with integration time - so longer subs should be more effective at improving S:N until you start saturating the sensor. Darks should get rid of the non-random (also called systematic) noise.
As other posters have pointed out, if the data is manipulated with sufficient precision in the computer (ie 32bit) it doesn't matter if you average or just add.
cheers,
Andrew.
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