Actually there is a difference, though some may not notice in practice. The pic below shows the relative darkness of the screens in four e-ink devices. The blacks (text) on each is the same, so the lighter screen has the greater contrast.
The enTourage eDGe (big one) and iRex DR800SG (top-middle) are about the same. The Kindle 2 (top-left) is darker than those and
much darker than the Cybook Gen3 (top-right). I went from the Cybook, my first e-ink device, to the Kindle and I was
so disappointed.
For my eyes, the Kindle 2 has too low of a contrast. You need a brighter light to read it. The iRex and eDGe are fine. And that Cybook ... it must be because it only has four levels of gray or something. The contrast is just outstanding.
It's possible that YMMV. Different lots of e-ink may have different characteristics. I don't know.