Short & Sweet:
1) According to the back of the box set, there are no bonus features (on any disc).
2) Prophecy II (2/Sequel), is NOT included in this "Collection", despite them all being made & distributed through Miramax (so there's no excuse of, "Don't have the rights", etc.)
If all they can give Prophecy fans, is somewhat better picture & audio (and even that is debatable, via Standard DVD w/ 'upscaling')... and none of the extras that were contained on the original DVDs (features, commentaries, etc.)... Not to mention, the 2nd Movie, which is after all, the springboard for the 3rd; & anyone unfamiliar w/ the series will have a huge gaping whole in the story-line as a result... Then why waste your money?
Keep your DVDs... and if you're really itching for an HD Experience - pop the DVDs in your PS3 or Blu-Ray player for the 'upscaling'... I'm sure it's just as good as what this is, considering they clearly didn't spend any time or effort on improving anything.
PS: If you are unfamiliar with the series, and don't own it on DVD already - Just buy the DVDs... they're (much) cheaper, include bonus features (on most), and you can get all 5 Movies (which is the most important thing). Plus, again, the quality won't be all that different anyway, if you have a DVD Upscaler, or if you play them in your Blu-Ray player.
Update: Read comments & my reply for more info.