SLS will increase visible noise. As the sensor is "on" for longer any noise it is producing will be allowed to accumulate for longer and thus become more visible.

That's an important point to keep in mind,
Thanks Alisatair!

By the way thanks Paul and Dave,
You're right and I know 1080p is cleary more detailed, so I guess i have to wait until I find a true HDTV monitor to test and watch footage.
Thank you guys for all our answers.

I am not sure this is right - noise is more visible when the ratio to clean signal is low- if the sensor is exposed for longer then the clean signal will be larger - the noise is pretty much a constant on the sensor - hence GAIN increases small clean signal (low light) AND the constant noise which then becomes visible.

So noise is a function of gain more than anything else

I'd have to disagree with you on this. Anybody who has ever shot a time exposure, for example, with a digital camera will start seeing noise and glowing "stuck" pixels as the length of time the sensor is exposed to light. So, my contention that SLS was one of the sources of his noise problems seems to make sense.....to me, anyway.

I think Alister is right. Assuming gain is unchanged, the same amount of light is needed to give a correctly exposed image whether normal or slow shutter is used. The difference is that with the longer accumulation time, the 'rate' at which that light needs to arrive is lower. Hence the greater sensitivity and, all other things being equal, the need to stop down as the SLS frames are increased. So the signal part of S/N is constant.

Assuming that noise accumulates over time, the longer the accumulation period, the more noise will be present. Constant signal and, with SLS, increasing noise suggests that the S/N will decrease.