The viewing angle is poorer in the MkII but you can play it with the screen "flat" in front of your eyes.
For a best color and brightness on Mk1 you should incline ahead the Gp2X a bit in order to have a better image.
Otherwise it could be a bit too dark for some games.
The contrasts are great on the MkII, perhaps a bit too saturated sometimes.
I like the render of my MkI, even if the colors can be a bit wasted sometimes. I also like the "scanlines" of it. Matter of taste.