What's better sand or gravel?

I have a 200 litre tank (about 45 litres I think that is) a lionfish a long spiked black Sea urchin, a short spiked sea urchin and 2 brittle stars. I have been struggling with high nitrates and have been doing bigger water changer that has got it down a bit but still have high nitrates. I hear if you have a sand bead you just need a good clean up crew to clean it because dirt dose not go as deep in sand as it would gravel. I did a clean of the tank and moved all my live rock what I don't always do in fear of burying a crab and there is a LOT of dirt coming up. What is peoples opinion on sand? I have crashed reef as a bed so I hear it's meant to be good but don't know that much about it. What do you use?