Unfortunately, this depends on the quality of the dithering. So you may not be able to find out for sure until the Nook is out.
The best thread I am aware of on this topic (read the entire thread) is More display issues on the DR
. This illustrates a) that the Sony's have excellent dithering, b) that 16 shades without dithering are "worse" than 8 shades with good dithering, and c) iRex has a buggy dithering implementation on the DR1000S. Note that except for (c) I would say that the iRex DR800SG is likely to be your best bet, although it is significantly more expensive than the Nook or PRS-600.