Well it depends on what type of discus. It is commonly believed that many of the pigeon blood discus which are prone to peppering will pepper worse with a dark background. The white sand y bottom will help, but ma not alleviate he problem. I personally have had blue, watermelon and black. I like the black best of all. The white sand looks very nice. That is what I have had with last two tanks.
Many hardcore Discus enthusiasts think a bare bottom tank is the only way to go. They especially think this when you have juvies that need frequent feedings and really clean water to reach their potential.
I have done barebottom and personally can't stand the look..but that is me.
In the end, look at tons of pics, decide what YOU like and go for t!!