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.