If you shoot raw, then stick with ISO100 and be prepared to "underexpose" then fix in post.
well that's not good advice!
Care to explain why shooting at ISO100 but underexposed by 2 stops is worse than shooting at ISO400 on a Nikon 800/600 series?
Numbers are useful here.
You have two main sources of noise at low ISO:
1) Quantum noise
2) Readout noise
Quantum noise is due to the fact that photons behave like particles and if you on average expect 10000 photons in a pixel in reality you get sometimes more and sometimes less leading to a Poisson distribution around that average value. The quatum noise is the square root of the number of photons e.g. 100 for 10000 photons and 10 for 100 photons.
The readout noise is various electronic noise and depends on the amplification factor and other interference. With the new Sony sensors the readout noise is virtually the same on ISO 100 to ISO 6400 e.g. 5 electrons. With the canon sensors the readout noise is better for high ISO e.g. 3 electrons but worse at low ISO e.g. 30 electrons at ISO 100.
If you look at a bright pixel you get 10000 electrons from the photons +-100 electrons due to Poisson statistics (even a perfect sensor will get that). The S/N ratio is 100.
If you look at a dark pixel you get 100 electrons from photons and +-10 electrons from Poisson statistics. The S/N ratio is 10.
Now factor in the read noise.
- Bright pixel:
-- Canon: 100 and 30 => 104 total, S/N 96
-- Nikon: 100 and 5 => 100 total, S/N 100
- Not much difference.
- Dark pixel:
-- Canon: 10 and 30 => 32 total, S/N 3.1
-- Nikon: 10 and 5 =>11 total, S/N 9.1
- Almost a factor of 3!
If you do the same calculation at high ISO, the Canon sensor gets a little advantage.
From this we can conclude that BRIGHT pixels have the SAME QUALITY with a Canon and a Sony sensor.
But DARK pixels suffer from readout noise and here the Sony sensor is much better.
So if you do not underexpose or lift the shadows in your pictures, you will be OK with current Canon sensors.
EDIT: Use RMS for noise addition.