In theory, DVD-Audio has the best potential sound quality. But in reality it comes down to the process, how well the music was recorded, mastered and authored. In some cases the DVD-A version sounds best, in others the SACD sounds better.
One advantage SACD does have over DVD-Audio is that it does not require a video component, as the DVD-Audio does specification. However, here are variations that get around this, such as the Digital Audio Disk and High Defintion Audio Disk formats. DAD is 24 bit/96KHz, HDAD is 24 bit/192KHz and both are 2 track stereo formats. DAD's can be played on regular DVD players, HDADs have to be played on a DVD-Audio player.
Another difference is that the SACD specification requires a 2 channel stereo layer, while the DVD-Audio spec is multi-channel only. A lot of DVD-Audio disks do have a stereo track, but it is often 24 bit/48KHz, rather than 24/96. And because it is not required in the spec, some DVD-A's don't have any stereo track at all.