For me, K2 is noticeably better at 1) navigating through web pages (because of the joystick) and 2) working menus (joystick again). The contrast of text-on-page is the same as far as I can tell, but the 16 step greyscale makes for better pictures (for blogs, etc) than the 4 step grayscale does. The scroll wheel has a faster response time, but the joystick provided (in my opinion) a better interface. The K2's page flip is slightly faster for some font sizes and spacings.
If you're interested I did a review of the K2 vs K1 here:
https://www.mobileread.com/forums/sho...073#post408073