HDCD doesn't add very much to the music. I've heard comparisons and it's hard to tell a difference (to my ears anyway). As Eug said, what you'll be looking to compare is SACD to DVD-A. Both offer multi-channel, high bitrate audio that sounds AWESOME. It finally bridges the gap between warm-sounding analog and harsh-sounding digital (CD et. al).
My money goes on DVD-A, just because of the "name recognition" aspect of DVD. Technically (and most audiophiles will tell you) SACD is the better format. But then again so was betamax, and look where it ended up. DVD-A also has more industry support than SACD.
But it will take a long time for either of these to catch on. Most consumers see nothing wrong with CD-DA and aren't looking to drop $$$ on a new format, especially when the selection is so limited.
But don't get me wrong, SACD and DVD-A both sound noticably better than CD-DA... the little nuances really stand out. Go have a listen at Circuit City or something. Then decide whether it's worth your money.
l2c