I said that the Shonen Jump cartridge is rarer than the prerelease, yes. I have both it and the prerelease cart on Ebay. Honestly, did you even read the auction listings??
As for the differences between the ROM and the final version, Im not sure if there's a hardware difference (I never cracked open the cartridge to check) but I assume there's at least moderate differences there. Big differences are in the software of the game itself, but as was posted earlier there's a whole huge section of the Compendium devoted to that.