In theory, the SACD version should sound better. In truth, it will depend on the mixing, mastering, authoring and manufacturing of the disks. When comparing very well mastered and produced redbook CDs to their SACD version, I have found very little difference. A good example are the re-mastered early Elton John CDs (Tumbleweed Connection, Madman Across The Water, etc.) compared to the SACD versions that were released last year. Gus Dudgeon (the producer) did such a great job re-mastering the redbook CD versions that there's very little difference between them and the stereo SACD versions.
Another factor may be the system. My system is decidedly mid-fi, a true high end system may make the differences more audible. And as always, each listener is different.