When I need to use brakes out in the dunes, I use both.
Front brakes are a must for me, rear brakes hardly do anything out in the dunes other than make it easier to swap ends.
The front brakes are very effective in the dunes, the nose dives down and the tires dig deep into the sand and plows to a stop...while the rear doesn't do much except for slide across the sand like a dog's azz on linoleum.