PA302W has a couple of advantages over PA301W.
It has GBr LED backlight with 2D brightness control, that can dimm G and B LED independently, so you can calibrate wtpt to mimic warmer photo papers without loss of tonal quality and contrast ratio (http://www.luminous-landscape.com/forum/index.php?topic=83820.msg699341#msg699341
The panel has slightly more gamut, which is noticeable - AdobeRGB coverage doesn't tell the whole story.
It also has RGB next to temperature sensor to control brightness and colour of backlight, so the display maintain stable for a longer period of time.
It has good brightness control, so it can be dimmed even to 20cd/m^2 natively, so you can comfortably work even in dark surround. The backlight brightness is controlled by 44MHz PWM, so there's no slighted trace of flicker BTW.
The display features satin type diffuser, so the image is cleaner and more paper-like than in previous generations with heavy matt diffusers, which suffered from clouding/sparkling effect.