The most important difference is that HD-DVD is a dead format and BD is still supported. There are many other technical differences, but they really don't matter any more.
HD-DVDs are incompatible with BD players and DVD players. You can only play HD-DVDs on hardware designed specifically to work with HD-DVDs.
Yes, HD-DVDs will look better than SD-DVDs played through your Integra receiver. You will need an HD-DVD player.
My recommendation is that if you aren't already invested in HD-DVD, just get a BD player instead.