SACD is a higher quality format than DVD-A (although they are both far superior to CD-A). SACD also uses Direct Stream Digital, which requires fairly new and VERY expensive recording equipment that not even all recording studios have yet. SACD is a proprietary, protected format so you really have to buy a disc already in that format to get it. Even some of the "SACD" discs are not "true", as they may not use DSD (they may pass through a high-end analog phase--true DSD is only analog on the original master, if then((new recording are pure digital, older ones are converted from superb analog master tape)).
My opinion is SACD is much like BetaMax--the better audio format, but it appears DVD-A may become more popular because it plays on a DVD player (only in Dolby Digital 5.1, which is very compressed crappy sound, but gives you surround sound). You still need a true DVD-A player to get the high-bitrate DVD-A, but many people seem happy with basically DVD sound from an audio CD (not even as good as CD-A, but in 5.1 surround).