Saying that "The Definitive Edition Series featuring state-of-the-art picture, superior sound quality, amazing bonus features, premium packaging and High Resolution Digital Transfer creating a spectacular look and feel."
Definitive Edition is called Special Edition in North America.
Usually it comes with a better audio track and features such as commentary on the movie, how it was made, and in some cases better video.
An example I have is Tora Tora Tora that has an optimal mode for better video and THX certified sound. Upscaled on my dvd player the picture is outstanding and so is the audio.
Whether it is worth the extra cost depends on whether you're a movie buff. I think the general public isn't concerned about buying movies, unless they're in the bargain bin, since we can rent them cheaper. And unlike music, videos may loose their appeal after viewing them a few times.
Unfortunately in the world of DVD there are no standards, which means that the quality level can shift dramatically from one title to the next, as well as from one edition to another, and even moreso from one country to another. DVD has certainly come a long way from where it once was in the beginning stages, but there are still DVD's being produced that just don't meet up to the level of expectation that people should have. Special Edition discs are often created for a few purposes: the first is to generate new life into a particular title, the second is to re-issue perhaps with additional features/footage/cuts, and the third is for audio/video updates/restorations/etc. These are the 3 main reasons why a title is re-issued onto the format, unless for another reason such as a factory flaw or something odd, these are the 3 most common.
Is the new edition better? In most cases...yes. In some cases, it's a toss up. Picture and sound should always be improved in re-issues, but that is not ALWAYS the case. Often times new features are added to the disc, which takes up more information, which leads to compression on the audio/video side. It's not that uncommon to see the picture decrease to some degree when more content is added to the disc. However, sometimes a new edition offers a high-definition transfer, or restoration that gives better picture quality and in some cases the audio has also been upgraded, in most cases the inclusion of DTS audio to counter the Dolby Digital tracks.
It would seem that you are buying discs from Austrailia, which means those discs are in the PAL format and have a tendency to be much sharper than the NTSC discs. I have only purchased one or two from that region, my copy of PICNIC AT HANGING ROCK as well as THE HULK.
My thought is that the Definitive Edition will probably be much superior to the basic edition in terms of creating a picture that is more like HD material, but still not near what HD-DVD or Blu-ray can offer. In many ways it's probably similar to the Superbit Edition of films that have been released worldwide.