I have an original mighty mouse, and the one that came with my new iMac. I honestly hadn't noticed any difference between them. The slight colour change on the side buttons is all that is different, and even then I didn't know until I saw this post, and checked.
To be honest, I think these mice are rubbish anyway. The scroll ball gets borked within a few weeks of use, and you cant open the things to clean/service them without near enough ruining it. Even more so the wireless version.