A couple things to consider:
1. Color: light sand can get algae and detritus in there and not look so white, black sand might not look so good if white pieces of rock and coral get mixed in over time.
2. Size: if you plan to "gravel vac" detritus out of the substrate, you won't want super-fine sand because it will get sucked up.
I'm not experienced enough to talk specifics about whether or not you would want to disturb the sand-bed based on its thickness, but I generally understand that one approach is to use a deep sand-bed to encourage anaerobic bacteria to grow and convert nitrates to harmless components and in this case you want to leave the substrate alone when cleaning the tank.
I'm using pretty fine grain size because I like the way it looks, but vacuuming it would be a problem. I am leaving it pretty thin (like 1/2" to 1") because I don't want it to get clogged with bad stuff. To clean, I just stir it up a bit to get the junk floating around in the water, then I vacuum the water out at about the middle of the tank (height-wise, I mean).