I have the green one and love it.
Imo, there's not a lot of difference between them except for the tactile differences. It pretty much comes down to whether you like/don't mind the feel of the black cover that comes with the eReader (I don't know what it's made of), or - in my case - whether a black covered eReader keeps getting misplaced in stacks of books and moleskines and other assorted flat black things while a green one is easy to find.
(Which I seem to have already mentioned...
Sorry about the repeat. Hey, at least I'm consistent.)
