The short answer
Do both, because a camp night gives you both. Sunset is the easy one: you ride out in the late afternoon and watch from a dune near camp. Sunrise is quieter, colder and, for many people, the best moment of the trip, before the wind rises and the heat builds. Photographers usually prefer sunrise.
In more detail
The timing of a desert night is built around the light. You arrive as the sun drops, so the sand moves from gold to deep copper. After dinner and a dark sky, the camp wakes you before dawn to climb the nearest crest and watch the sun come up over the dunes. Our desert guide calls that sunrise the moment most travellers name as the highlight of their whole trip.
Sunrise has fewer people, cleaner sand with fresh wind ripples, and long shadows across the dune shapes. It is also the coldest hour, so wear everything. Sunset is warmer and more social, and the light lasts a little longer as you ride.
Climbing a big dune is hard work in soft sand, so start early and go slowly. Barefoot is easiest in mild months; in winter the sand is cold before sunrise. If you only manage one, our pick is sunrise.