I have a Kindle 1 but I almost never use it, preferring my EZ Reader for routine use. However, I typically use the OpenInkPot 3rd party firmware, although the standard firmware is also good enough. The screens for all the 6" EInk devices are the same, the K2 has 16 greyscales (vs 4 on the EZ Reader) and this might matter a bit for anti-aliased fonts. I would guess that the K2 is slightly faster for page turns, because it has the new Epson screen controller. See
BeBook font sizes and types for scans of the EZ Reader (these are scans at lower resolution than the screen itself). I have the original EZ Reader, but the new ones have a smaller logo on the top (they are otherwise identical), see
The logo on your EZ reader?.