I am also considering both of the current Panasonic Plasmas to replace my old Panasonic SDTV plasma. You are correct that they list the contrast ratios as 20,000 for one and 30,000 for the other, but I believe they list the dynamic contract ratio of both as 1,000,000. Reviews I've read said you can see the difference , especially with dark backgrounds.
The other differences between the two are PC inputs for the more expensive model and better cosmetics for the more expensive unit (subjective).
I think I'm going with the more expensive model so I can use the TV as the monitor for my future PC based digital music playback system.