Coming back to the topic...
I have all 3 players, an upconverting Oppo, an HD DVD and Blu-Ray players.
The Oppo upconverting and universal player provides me with better video for SD DVD than the 2 others. But this picture quality is dependent on the quality of the DVD. Recent SD DVD releases are vastly improved over those of a few years ago. Therefore, a recent release for SD DVD can satisfy many.
But, the picture quality does not match those of my HD players. The close up shots in HD is better but the main difference is the resolution of background scenes which is clear in HD and blurred in SD DVD and even the Oppo cannot correct that.
And now I rent HD DVD and Blu-Ray movies from a mail in rental service and rent an occasional SD DVD from a local rental service. The mail in rental service rents the HD movies for the same price as for SD DVD. So, I only rent HD movies from them.
HD movies and HD concerts, a particular interest for me, offer better audio... lossless audio. And that in itself, is reason enough to get an HD player which can decode lossless audio. And that's what I have. Elton John 60 in Blu-Ray should arrive in the next days and Celine Dion's "A New Day", also in Blu-Ray should get here in February. The first one has uncompressed PCM audio.