[patrickg_zill]

This is a hotly debated issue on the Sigma forum I linked to.

The "Merrill" sensors (named in honor of Dick Merrill, an engineer and photographer that did a lot of work on that generation of sensor) are 1:1:1 as you mention - each of the photosites pick up all 3 colors.

The "quattro" sensors have the top, blue-sensitive, layer at the full resolution, with this layer also capturing the luminance of the image (which makes for stunning black and white, even at ISO 3200). The lower 2 layers are at a lower resolution than the blue layer.

I did get very high quality 13x19 inch prints from the quattro ; but have never owned a camera that used the previous Merrill sensor, so I can't give a direct comparison from my own experience.