I own five 6" screen devices (Kindle 1, Kindle 2, Cybook Gen 3, eSlick, and the Kobo) one 8" screen (Irex Iliad) and two 9" screen (Kindle DX and Entourage Edge).
All the screens come out of the same manufacturing plant. The screens show show some variation where the "white" background is slightly whiter on some screens but the "blacks" are also slightly less black. All of the screens have the same contrast ratio and that is not affected by the display controller. All the controller can do is give a fixed number of shades of gray. IIRC, the Kindle one has 4 shades, the later Kindles and Sonys have 8 shades. That doesn't affect the contrast just the number of gray tones that can be rendered.
Firmware also plays a huge part in how well the screen renders text. On my Kindle 2 I added the font hack as soon as it became available because I thought that the text was too hard to read, the letters too thin. On my DX the presumably same original font is perfectly readable. Is that a difference in firmware or the screen itself? I don't know but when I compare my three Kindles the Kindle 2 screen is not quite as good as the other two. But I've seen other Kindle 2s with screens that I thought were better than mine.
The screens with better contrast are not in production and I have no idea when that will start.
I'm not exactly an ereading device hoarder. I've gifted family members with a good part of my collection.
I'm still looking for the perfect device.