HDMI is just the interface that will transport your audio & video digitally from your DVD player to the TV. This is compared to component, which send the signal in as an analog waveform, which must then be converted back to digital by the TV. HDMI keeps the signal digital all the way to the TV.
As for the quality of the upscaling between various DVD players, this has nothing to do with the HDMI, but is dictated by the quality of the video processing electronics in the players. The best way is to simply do a side by side comparison and judge for yourself.